5 wholes = 5
2 fifths = 2/5
5 - 2/5 = 4 3/5 (4 and 3 fifths)
Answer = 4 3/5
Check work:
4 3/5 + 2/5 = 5 (wholes)
You need to have variables one one side and constants on the other. ax - ax + by = c - ax
by = c - ax
by/b = (c - ax)/b
y = (c - ax)/b
$30.50-$17.79 is $12.71. I hope that helps you :)