Adrian has $6.35 in dimes and quarters. The number of dimes is three more than three times the number of quarters. How many quarters does he have? What expression represents the number of dimes if q represent quarters

I: 635=dimes*10+quarters*25

or shorter

635=d*10+q*25

II: d=q*3+3

Substitute II (d=3q+3) into I to replace d:

635=d*10+q*25

635 = (3q+3) * 10+q*25

635=30+30q+25q

605=55q

11=q

-> 11 quarters

"What expression represents the number of dimes if q represent quarters"

insert q=11 into II:

II: d=q*3+3

d=11*3+3

d=33+3

d=36