Novak needed to dig up three of the same pit in the garden. The first pit was dug father alone for 15 hours. His second dig son helped him, and it did that in six hours. The third pit was dug son himself. How long does it take him?

Correct answer:

x =  10 h

Step-by-step explanation:

a=15 h 6 (a1+x1)=1 a1+x1=61  x/a+1=x/6 x/15+1=x/6  3x=30  x=330=10  x=10=10 h

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