Well, for number 15, this is what I came up with, I hope it is what your teacher is looking for:
So it starts out with 3 butterflies, and it doubles every minute:
3x2 = 6 That is for the first minute
6x2 = 12 That is for the second minute
12x2 = 24 That is for the third minute
24x2 = 48 That is for the fourth minute
So, It asks for using the power of 2 in your answer, that is what I came up with for part A.
For part B I would say something like this:
X represents the number of butterflies left.
The equation, using the information above would be this:
48-7 = X
Then simplify, which would leave you with 41 butterflies.
I hope this helps! :)