Question 600419
number of quarters coins = q
number of half-dollars coins = h


q + h = 18


1/4 q + 1/2 h = 7.5
multiply all by 4:
q + 2h = 30


use the elimination procedure:
q +  h = 18
q + 2h = 30
------------ (-)
-h = -12
h = 12


q + 12 = 18
q = 18 - 12 = 6


So he has 6 quarters