Question 370210
Let Q be the number of quarters, D dimes.
1.{{{Q+D=20}}}
.
.
{{{25Q+10D=320}}}
2.{{{5Q+2D=64}}}
Multiply eq. 1 by -2 and add to eq. 2,
{{{-2Q-2D+5Q+2D=-40+64}}}
{{{3Q=24}}}
{{{highlight(Q=8)}}}
Then from eq. 1,
{{{8+D=20}}}
{{{highlight(D=12)}}}