You can find a unit rate by dividing the first number by another number to get something per something.
For example, if a question asked: What is the unit rare for 100 feet for 2 seconds, it would be 100/2, so it is 50 feet per second.
Mowers =6, tillers = 3, rent = $19
Not rented = 2 mowers, 1 tiller
(4+2) x 19
6 x 19
$114 was collected that day