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There is a math problem.
Arithmetic solution: 48-36= 12.

24- 18=6

∴ sailing downstream12km = sailing upstream for 6km.

It takes seven hours for a ship to sail 36 kilometers downstream and 24 kilometers downstream.

It takes seven hours for a ship to sail 84 kilometers downstream.

The speed of the ship sailing in the water is12km/h.

Similarly, the speed of sailing against the current is 6 km/h.

∴72/ 12+24/6= 10

It takes 10 hour to sail 72 kilometers downstream and 24 kilometers upstream.

Equation solution:

Let the downstream speed be x and the upstream speed be y.

Then 36/x+24/y=48/x+ 18/x=7.

∴x= 12 y=6

∴72/ 12+24/6= 10

It takes 10 hour to sail 72 kilometers downstream and 24 kilometers upstream.