<span>9.3181819e+12v is the answer</span>
Answer:

Step-by-step explanation:
The systems of equations are
Let us assume the rate of the current be c
Now actual speed against the current is (15 - c)
And, the actual speed with the current is (15 + c)
Here we applied the time formula i.e. distance by speed
Also the upstream time - downstream time = 4
Now the equations are

For all of them you can just simplify to their lowest fractions like this and they'll still be equivalent 2/10, 1/4, 9/10