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Ответ:

S - расстояние м/у A и B

v2=1.4v

t2=0.7t

x - скорость течения

(S/(v+x))/(S/(1.4v+x))-?

S/(v+x)+S/(v-x)=t

S/(1.4v+x)+S/(1.4v-x)=0.7t

Поделим (1) на (2)

(2v/(v²-x²))/(2.8v/1.96v²-x²)=1/0.7

(1.96v²-x²)/(v²-x²)=2

1.96v²-x²=2v²-2x²

x²=0.04v²

x=0.2v

Далее

(1.4v+0.2v)/(v+0.2v)=1.6/1.2==4/3=1 1/3