First you need to find the common denominator. Think of a number that both 2 and 5 can go in to evenly. 10 is the best number, so, make both denominators 10. What you do to the bottom, you must do to the top.
Now your equation is 5/10-2/10
Your answer is 3/10
2^n = 1/8
Try each one of the options in turn
The question marks are negatives right?
Answer:
Speed of boat in still water is 6km/h
Speed of current is 4km/h
Step-by-step explanation:
We know speed = distance / time
Let the speed of boat in still water be x
Let the speed of current be y
Hence the speed of boat, down stream = 
Distance downstream = 20km
Time taken = 2 hours
We have

......(1)
Speed of boat upstream = 
Distance upstream = 4
Time taken = 2
We have

...(2)
Adding (1) & (2)


Substituting in (1) we get

V=pi a^2*h
V= 3,14*100*18 cm^3=5652 cm^3
Answer is 5652