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?

A)$11
B)$11.50
C)$12
D)$12.50

Answer 1

11*57+X(1+2+3+.....+56)
=11*57+X(1+56)*56/2
=627+1596X
=19779

X=(19779-627)/1596=12
Your answer is C

Answer 2

we can translate the following statement into a1 equal to $11, n equal to 57, and s equal to $19799. The formula to be followed through derivation is 
s = (n/2) * (a1 + a1 + d* (n-1))$19799 = (57/2) * (2*11 + d*56)d = $12.01
The answer that is closest is option C