[tex]3\ pumps\ work\ 6\ hours\ so\ summing\ work\ each\ of\ them\ they\ work:\\
3*6=18hours\ in\ one\ day\\\\
18*2\ hours\ is\ needed\ to\ empty\ water\ tank\ in\ 1\ day\\\\
9*x\ hours\ is\ needed\ to\ empty\ water\ tank\ in\ 1\ day,\\ where\ x-\ number\ of\ hours\ during\ which\\ every\ of\ 9\ pumps\ work\ in\ one\ day\\\\
Making\ ratio:\\
18*2\ hours-1\ days\\
9x\ hours-\ 1\ day\\\\
36=9x\ \ |:9\\
x=4\ hour\\ 9\ pumps\ must\ work\ 4\ hours\ per\ day\ to\ empty\ water\ tank\ in\ one\ day.
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