Write and solve a system of equations for this question

Let b and s be the velocity of the boat and stream respectively

3(b-s)=27

b-s=9

b=9+s  we will use this in the second equation

2(b+s)=30

b+s=15, now use b found above in this to get:

9+s+s=15

2s+9=15

2s=6

s=3, and since from earlier b-s=9, b=9+s so

b=12

So the speed of the boat is 12mph and the stream is 3 mph