The principal is $25 and the rate is 4%.
So after t years: A(t) = 25(1.04)^t
I uploaded the answer to a file hosting. Here's link:

You just need to foil. 2x times 4x times 2x times 36 times 24 times 4x times 24times 36. then solve.
Answer:
9 years old.
Step-by-step explanation:
Since the ages of all of these students are being represented in relation to the middle student's age then we can create formulas for each student and then apply them to the formula for the sum of their ages and solve for the middle child's age (M) like so... (oldest child's age is represented by variable O and youngest child's age is represented by variable Y)
O = M + 6
Y = M - 3
30 = (M+6) + M + (M-3) ... combine like terms
30 = 3M + 3 ... subtract 3 from both sides
27 = 3M ... divide both sides by 3
9 = M
Now we know that the middle child is 9 years old.