Ajar contains half dollars and quarters there are 30 coins in the jar. the total value of the coin is 8.75. how many half dollars are in the jar?
lesson is: application of system

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