Do you know what you need to do to solve this problem?
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

Cost = fix cost + hourly cost - discount
Cost = 4* [5 + 4x - 4x(20/100)] = 4*[5 + 4x - 0.8x] = 4*[5 + 3.2x] = 20 + 12.8x
Answer: option B) 12.8x + 20
5*2 = 10
10*1 = 10
Hope I helped