let h = the number of half dollars in the jar
and q = the number of quarters
h + q = 30
0.5h + 0.25q = 8.75
multiply the second equation by -2 to get
-h - 0.5q = -17.5
Now add the new equation and the first to eliminate h
h + q = 30
-h - 0.5q = -17.5
----------------------------
0.5q = 12.5
q = 25
There are 25 quarters, so there must be 5 half dollars
