q=number of quarters
d=number of dimes
so the value of 1 quarter=0.25 and 1 dime=0.10 so therefor the total value would be
0.25q+0.1d=2.9
total number of coins=20
q+d=20
solve
we have
0.25q+0.1d=2.9 and
q+d=20
multiply the first equation by 10
2.5q+1d=29
so we have
q+d=20
2.5q+d=29
multiply first by -1 and add
-q-d=-20
2.5q+d=29 +
1.5q+0d=9
1.5q=9
multipliy both sides by 2
3q=18
divide by 3
q=6
subsitute
q+d=20
6+d=20
ubtract 6
d=14
14 dimes
6 quarters
