Water usage and output

Question

The the Robinson family and Smith family each used their sprinklers Last Summer the water output rate for the Robinson family sprinkler was 40 liters per hour the water output rate for the Smith family sprinkler was 30 liters per hour the family's use their sprinklers for a combined total of 40 hours resulting in a total water output of 1350 L how long was each sprinkler used

Answer 1

Let S and R = the Smith & Robinson water use respectively

S + R = 40

30S + 40R = 1350 => 3S + $R = 135

It is a toss up whether to use elimination or substitution to solve...you pick the method and solve it.