Question 1110094
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.