Question

Jennifer has been saving for college for 57 months. The first month, she saved $11. She was able to save more money each month than the month before. She ended up saving $19,779.00. How much more did she save each month?

Answer 1

This is an arithmetic sequence with first term (a) = 11, number of terms (n) = 57, sum of nth term (Sn) = 19,779.
Let the amount of money she saved more each month be d, then
Sn = n/2(2a + (n - 1)d)
19,779 = 57/2(2(11) + (57 - 1)d)
19,779 = 28.5(22 + 56d)
19,779 = 627 + 1,596d
19,779 - 627 = 1,596d
d = 19,152/1,596 = 12

Therefore, she saved $12.00 more each month.