First of all, it is certain that the two trips are exactly the same, both of which are 2S. For the first time, a ship with t 1=2S/V was used; (Type 1)

The second time, t2= S/(V boat +V water) +S/(V boat -V water)

Variant: divide the numerator denominator by the V-boat first, and you can get:

T2=2S/[V ship -(V water 2/ ship)] (Equation 2)

Comparing the two formulas (1) and (2), we can see that the numerator of the two formulas is the same, but the denominator of the formula (2) is small, so t2 is greater than t 1.