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Xiaoshengchu's math application problem is extremely difficult.
It takes a day for Team A and B to complete a tunnel project. How many days does it take if two teams work at the same time?

Solution: If the total project amount is 1, then Party A will complete 1/a every day and Party B will complete 1/b every day, so the number of days required for simultaneous construction of two teams is

1/[( 1/a)+( 1/b)]=(a×b)/a+b)

The distance from port A to port B is S. If the speed of a ship from port A to port B is U 1 and the speed of returning from port B to port A is U2, what is the average speed of the ship?

According to the meaning of the question, the time for a ship to sail from Port A to Port B is S/U 1, and the time for the port to return to Port A is S/U2, so the average speed is

2S/[(S/U 1)+(S/U2)]

=2(U 1×U2)/(Ui+U2)