Question 875057
Uniform rates situation, {{{R=v/t}}}, R is a constant for each agent; v is volume in liters and t is time in hours.  The rule for this uniform rates idea is equivalently {{{Rt=v}}}.


Petersons:  Let P = sprinkler rate.  
Volume was P*20 liters.


Gonzalezes:  Let G = sprinkler rate.
Volume was G*40 liters.


Combined volumes: {{{highlight_green(P*20+G*40=1800)}}} liters.


Also given was that the combined rate for the two families' sprinklers was 55 liters per hour.  This means, {{{highlight_green(P+G=55)}}}.


This feels like an unrealistic thing, because the two different sprinklers probably did not sprinkle simultaneously - even so, we have two equations and two unknown variables, P and G.  Simplify the combined volume equation to {{{P+2G=90}}}, and we have the system:
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P+2G=90
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P+G=55
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Solve that system for P and G any way you want.  Elimination would be a good way to start.  G=35 and so P=20, liters per hour.