Jar contains 51 coins which are either quarters or nickels. total value of jar is 5.15. how many of each type of coin do they have.

I believe there are 20 quarters and 3 nickels

Total value of the jar is $5.15

Quarters are worth 25 cents
And nickels are worth 5 cents.

First you know that there are 4 quarters in a dollar.
 You have 5 dollars so you do 4 x 5 = 20

So five dollars ($5.00) is 20 quarters.
And you still have 15 cents left.

Because quarters are 25 cents the rest are nickels.

Nickels are worth 5 cents so you'd do 5 x 3 = 15.

15 cents!

Add all this together and get $5.15.

So your answer is 

There are 20 quarters and 3 nickels in the jar.