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?

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

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