Lauren used 3.6 gallons of water to water her plants over 9 weeks. If she used the same amount of water each week, how many gallons of water did Lauren use each week?

Summed up: Lauren used 3.6 over 9 weeks. How much did she use per week?

3.6/9 = 0.4 gallons

We're taking the length of time and dividing that into the quantity used to see how much was used each week.