Question 324768
A piggy bank contains $19.75 in quarters and dimes equaling a 100 coins in all.
 How many dimes are there?

Let x = no. of dimes
then, since the total no. of coins is 100:
(100-x) = no. of quarters
:
.10x + .25(100-x) = 19.75
:
.10x + 25 - .25x = 19.75
:
.10x - .25x = 19.75 - 25
:
-.15x = -5.25
x = {{{(-5.25)/(-.15)}}}
x = +35 dimes
:
:
Check solution, no. of quarters: 100-35 = 65 
.10(35) + .25(65) = 
3.50 + 16.25 = 19.75