Sally and Peter have a lawn-mowing business. They earn $30 for
a large yard, $21 for a medium-sized yard, and $15 for a small
yard. If they mow 8 yards and there are at least 2 yards of each
size, what is the maximum amount of money they can earn?
x = no. of small yards
y = no. of medium yards
z = no. of large yards
Naturally, they make the most money when mowing the greatest number of large yards.
But, x>=2 and y>=2.
So, x+y >= 4
If the mow 8 yards, then z = 8 - (x+y) <= 4
mowing 2 small, 2medium, and 4 large yards, they earn
2*15 + 2*21 + 4*30 = 192