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dirk can clean a playground in 5 hours. geralyn can clean a playground in only 4 hours.

which of the following can be used to determine the amount of time it would take for dirk and geralyn to clean a playground together?
a.) (1/5) + (1/x) = (1/4)
b.) (1/x) + (1/4) = (1/5)
c.) (1/5) + (1/4) = (1/x)
d.) (1/4) + (1/5) = (x/9)