An empty swimming pool is filled by two inlet pipes. one pipe can fill 1 110110 of the swimming pool in one hour. the other can fill 1818 of the swimming pool in one hour. if both inlet pipes are used to fill the pool, how many hours will it take to fill the empty pool

Let's say one pipe can fill 1/10th of the swimming pool per hour; and the other can fill 1/8th of the pool per hour. Then, working together they can do:
1/8+1/10=5/40+4/40=9/40 pool per hour
1 pool/(9/40) pool/hour=40/9 hours for one pool when the pipes are working together
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3cm and 6 and 9 cm that a possibility of that question