Sally has $21.40 in dimes and quarters. There are 100 coins in all. How many of each coin does she have?

x = dimes

y = quarters

We also know:

Dimes: 0.10

Quarters: 0.25

100 = x + y

So, 0.10x + 0.25y = 100

And, y = 100 - x

Let's use substitution:

0.10x + 0.25 (100 - x) = 100

= 0.10x + 25 - 0.25x = 100

= 25 - 100 = 0.25x - 0.10x

= (-75) = 0.15x

x = (-75) divide by 0.15

x = (-500)