61. Mowing and shoveling. When Mr. Wilson came back
from his vacation, he paid Frank $50 for mowing his lawn
three times and shoveling his sidewalk two times. During
Mr. Wilson's vacation last year, Frank earned $45 for
mowing the lawn two times and shoveling the sidewalk
three times. How much does Frank make for mowing the
lawn once? How much does Frank make for shoveling the
sidewalk once?
solve by substitution