Question

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

Answer 1

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