q = quarters, d=dimes.
q+d=30
.25q+.1d>5.20
Solve the first equation for d in terms of q.
d=30-q
Substitute that for d in the second equation.
.25q + .1(30-q)>5.20
.25q +3 -.1q <5.20
.15q>2.20
q>14.67
At least 15 of the coins are quarters.
We can solve the first equation for d, but it's clear just by looking that d=15 when q=15. That means at most 15 of the coins are dimes.
Check my profile and contact me for one on one online tutoring. I can
help you understand this with one on one online tutoring. It's one thing to
follow an example, but even better if you understand the concept.
(