Question

I'm saving money over a year to buy a $157.50 gift. I start with an empty piggybank. I put in a small amount of money. Each month after January, I put in $1.75 more than I put in the previous month. By the end of the December I've saved $157.50 exactly. The amount of dollars that I had in my piggy bank after April's deposit was...?

Answer 1

You have an AP with
a = ?
d = 1.75
S12 = 12/2 (2a + 11*1.75) = 157.50
Now just solve for 3.50
In April,
S4 = 5/2 (2a = 4d)