Yes, this is correct. Having no destroyers is a pretty big mistake.
Best attack

what would be the most effective attack against 10 infantry, 1 artillery, 1 armor, 1 fighter,
11 bombers, 1 armor, 1 infantry, cost 130 I.P.C.
5 infantry, 5 artillery, 5 armor, cost 60 I.P.C.

It depends what you consider ‘effective’. If effective means higher chance of success, then the first option is better. If it means lower losses  IPCwise  then the second option is better. To give values:
11 bmr, 1 arm, 1 inf  94% chance of victory (assuming at least 1 land unit kept to take territory), expected IPC loss of 75 (1 inf, 6 bmr)
5 inf, 5 rtl, 5 arm  85% chance of victory, expected loss of 31 IPC (5 inf, 4 rtl)Of course, the second option is more reasonable, since you lose less to gain the territory and you have units to defend in subsequent turns.

thank you,
20 armor for 100 I.P.C. could be a good option,
a sure better % chance of victory rather than the 60 I.P.C. option

A fairer question to pose would involve both options spending the same amount of IPCs.
I can assure you that $130 spent entirely on a mix ground units would fair much better than the 11 bomber option.
The trick would be to find the optimal mix of INF, RTL and ARM for the $130 budget

@DY:
The trick would be to find the optimal mix of INF, RTL and ARM for the $130 budget
And as I am sure you guys know, that would be the 411 doctrine.
4 infantry, 1 artillery and 1 tank.
I belive that 90 % of the A&A community favour this combo. I know I do. But this is off topic, since the question was not about the ultimate mix of units, but what was best of 11 bombers or 5 inf, 5 art and 5 tanks.

Yes, but the original question is ridiculous because it is comparing a single apple with a bag of oranges, and basically asks which choice is more likely to fill you up :?

what would be the most effective attack against 10 infantry, 1 artillery, 1 armor, 1 fighter,
11 bombers, 1 armor, 1 infantry, cost 130 I.P.C.
5 infantry, 5 artillery, 5 armor, cost 60 I.P.C.
I would say that this pretty much depends…
It is a tricky question, with no obvious answer.Given that there are no AntiAir gun present :
11 bombers/1 tank/1 art have a 94 % chance to success.
5 inf/5 art/5 tanks have a 85 % cance to success.The 11 bombers mix are the most effective when the object is to just utterly destroy the enemy force, no matter the IPC cost. This could be a case where your allies are supposed to go in and secure the territory after you have destroyed the enemy. This may also be a strafe attack, whit one round of combat only, and you want to retreat your surveving units as far away as possible. This could also be an amphibious assault, and you figure 11 bombers are cheaper than buying trannies for your men. It could also be a situation where you need your firepower in a spot far away ASAP, and bombers do move faster than infantry.
The 5/5/5 mix have less chance for success, but are cheaper to use, as our friend Uberlager stated in his outstanding post. The expectet loss for the bomber mix are 75 IPC loss, against only 31 IPC loss for the 5/5/5 mix. That is a lot difference, man.
If it was my call, I would send a mix of 10 infantry, 3 artillery and 4 tanks. But that is just me.

@DY:
Yes, but the original question is ridiculous ….
Not neccessarily, my friend. If your object is to take the last VC, wich happen to be 4 spaces away from your factory, and win the game, then 11 bombers may be a better choice than 40 infantry. Everything depends. Speed, speed and more speed is vital in this game.

Wow am I good at this. Next thing you know I am wrighting Strategy papers. You better watch out now, man.

@DY:
Yes, but the original question is ridiculous ….
Not neccessarily, my friend. If your object is to take the last VC, wich happen to be 4 spaces away from your factory, and win the game, then 11 bombers may be a better choice than 40 infantry. Everything depends. Speed, speed and more speed is vital in this game.
The question is ridiculous because it’s basically asking if two guys go to the store and one guy has $60 and the other guy has $130, who is going to come back with more in their shopping basket.
He should compare 2 equal budgets to give a fair assessment.
As you say, there will sometimes be tactical reasons to choose one purchase over the other, but can’t you see the premise behind the OP has a fundamental flaw?

Oh Yeah, here is a fundamental flaw from the OP yuo can’t argue against
11 BMB 1 ARM 1 INF = 140 IPCs (not $130)
So if mobility is key, how about with the extra $80 to spend, I add 6 BMB 1 INF 1 ARM (or 8 FTR) to the original unit mix.
I don’t have a dice sim handy, but I’m sure the win % would be approaching 100%

Both attacks ($80 on BMB or FTR as above) round off to 100% in fact.

@DY:
The question is ridiculous because it’s basically asking if two guys go to the store and one guy has $60 and the other guy has $130, who is going to come back with more in their shopping basket.
He should compare 2 equal budgets to give a fair assessment.
The IPC budget is not neccessarily the big point. In this game the limit is the numbers of new units that can be placed at a factory. So if you got a placing limit of 10 new units, and 120 IPC to spend, then 11 bombers are a better choice than 5 infantry and 5 artillery. But if you only got like 38 IPC to spend, then 11 bombers are out of the question.
I will state that every unit is worth their cost. You will always get what you pay for. What units to choose all depends on the situation on the map, period. If you play USA in the endgame and need one more VC to win, then 11 bombers will be a better buy than 40 infantry.

If you have $120 to spend, chances are you’ve won the game anyway.