The Alexander family and the Thompson family each used their sprinklers last summer.

Question

The water output rate for the Alexander family's sprinkler was 40L

 per hour. The water output rate for the Thompson family's sprinkler was 30L

 per hour. The families used their sprinklers for a combined total of 70

 hours, resulting in a total water output of 2400L

. How long was each sprinkler used? 

Answer 1

While the above equation is correct, it's necessary to explain how we get there.

The Alexanders used 40L/hr for x hours.

The Thompsons used 30 L/hr for y hours.

Their combined total number of hours is x+y=70.

Their total water output was 40x+30y=2400.

Using substitution, y=70-x from the 1st equation is used to rewrite the second equation

40x+30y=2400

40x+30(70-x)=2400

From there, distribute, combine like terms, and solve for x.

Once you have a number for x, substitute it back into the first equation

y=70-x to find y.

Answer 2

40*x +30*(70-x)=2400