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Math problem: the fourth grade of primary school! ! ! Urgent! ! ! Seek methods and processes! ! ! Thank you mathematicians! ! ! ~
The passenger ship takes 2 hours.

20× 2 = 40km

When the cargo ship set out, the two ships separated.

180-40= 140 (km)

There are two ships sailing every hour.

20+15 = 35km

The cargo ship ran for several hours and met a passenger ship on the way.

140÷35=4 (hours)