Question 169934
Let 
b be average speed of boat, 
c = average current.
gives
2(b+c)= 36
3(b-c)= 36
expanding
..
2b+2c=36....divide by 2, b+c=18 giving 3b+3c=54
3b-3c=36..given
..

3b+3c=54...eqn 1
3b-3c=36...eqn 2
subtract eqn 2 from 1
6c=18
c=3
b=15
...
Answer:
Rate of boat = 15 mph
Current      =  3mph