L = hours used by the Lopez's sprinkler
R = hours used by the Russell's sprinkler
so, we know the Lopez's sprinkler uses 15 Liters per hour, so say after 1 hour it has used 15(1), after 2 hours it has used 15(2), after 3 hours it has used 15(3) liters and after L hours it has used then 15(L) or 15L.
likewise, the Russell's sprinkler, after R hours it has used 40R, since it uses 40 Liters per hour.
we know that both sprinklers combined went on and on for 45 hours, therefore whatever L and R are, L + R = 45.
we also know that the output on those 45 hours was 1050 Liters, therefore, we know that 15L + 40R = 1050.
how long was the Lopez's on for? well, L = 45 - R.
R = hours used by the Russell's sprinkler
so, we know the Lopez's sprinkler uses 15 Liters per hour, so say after 1 hour it has used 15(1), after 2 hours it has used 15(2), after 3 hours it has used 15(3) liters and after L hours it has used then 15(L) or 15L.
likewise, the Russell's sprinkler, after R hours it has used 40R, since it uses 40 Liters per hour.
we know that both sprinklers combined went on and on for 45 hours, therefore whatever L and R are, L + R = 45.
we also know that the output on those 45 hours was 1050 Liters, therefore, we know that 15L + 40R = 1050.
how long was the Lopez's on for? well, L = 45 - R.