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I think the answer is 66 because they have the same amount candles as the age that Ben is turning for 11 years straight. Therefore, you need to add all the numbers from 1 through 11. As it says including his 11th birthday.
Now on to the what if question...
From my perspective I think each candle lasts 2 years. Owing to this fact you will just have to divide by 2 as there are 2 times less candles. So in this case they will only need 33 candles.
On the other hand, this question can be impossible because the problem does not tell you how long the candles last.
I made my own what if questions. You can try to work them out:
What if Ben had a twin or triplet?
What if Ben had a younger sister?
What if Ben had 2 birthdays because he is royal like the Queen?