Question

You use 4 gallons of water on 30 plants in your garden. At that rate how much water will it take to water 45 plants?

Answer 1

The easiest way to do this is with cross-multiplication.

4 gallons for 30 plants is written as 4/30

x gallons for 45 plants is written as x/45

Multiply 4 x 45 = 180

divide by 30

= 6 gallons of water

Answer 2

4/30 = x/45
cross multiply 
180 = 30x
divide
x = 6 gallons