If greg has 6 quarters and 15 pennies. how many total coins do greg and his brother have

Simple math problem here! :)

You add.

6 + 15 = 21 coins.

But, if you meant how much money do they have total the answer would be as followed. 

1 quarter = 25 cents
So 6 * .25 = 1.50 
(1 dollar and 50 cents)

1 penny = 1 cent
So 1*15 = 15.

Now you add!

1.50 + 15 = 1.65
(1 dollar and 65 cents.)

Hope this helps. c: