SOLUTION: the parents of a 9-year-old boy have agreed to deposit $10 in their son's bank account on his 10th birthday and to double the size of their deposit every year thereafter until his

Question 1110094: the parents of a 9-year-old boy have agreed to deposit $10 in their son's bank account on his 10th birthday and to double the size of their deposit every year thereafter until his 18th birthday.
a) how much will they have to deposit on his 18th birthday?
b) how much will they have deposited by his 18th birthday?
Answer by addingup(3677) (Show Source): You can put this solution on YOUR website!
a = 10 and r = 2
a_1 = a = 10 (his age, he is 10 at the time of the 1st deposit)
a_2 = ra = 2 x 10 = 20 (he is 11)
a_9 = r^(9-1) x a = 2^8 x 10 = 2,560 this is how much they deposited on his 18th b'day.
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By his 18th b'day they will have deposited:
S_9 = [a(r^9-1)]/r-1 = [10(2^9-1)]/2-1 = 10(512-1) = 5110 this is how much money the parents have deposited by his 18th b'day.

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