Question 350987
a coin collection contains nickels dimes quarters there are 1.) twice as many dimes as quarters and 2.) seven more nickels than dimes 3.) the total valve of all the coins is $3.10 how many quarters are in collection

Let n be the number of nickels, d the number of dimes and q the number of quarters. From 3) we have:

1.) (5*n) + 10*d + 25*q = 310 

We know from 1.) that  d = 2*q. From 2.) we know that n = d+7 so n = (2*q) + 7. 

Substituting in 1.) we have then:

5*((2*q)+7)) + 10*(2*q) + 25*q = 310  

10*q + 35 + 20*q + 25*q = 310

55*q = 310 - 35
55*q = 275
q = 5