**Concept: The question tests on the application of Relative Speed**

**Solution:**

Average speed over 2 equal distances when traveling at speeds a and b is given by 2ab/a+b

In this case let a be the speed downstream and b be the speed upstream

Let B be the speed of boat and S = speed of stream = 10

Speed upstream = B - 10 and Speed downstream = B + 10

Then Average Speed = 2∗(B+10)∗(B−10)/(B+10)+(B−10)=48

2∗(B2−100)/2B=48

B2−100=48B

B2−48B−100=0

B2−50B+2B−100=0

B(B - 50) + 2(B - 50) = 0

(B + 2) (B - 50) = 0

b = -2 and B = 50

Therefore Speed downstream = B - 10 = 50 - 10 = 40**Option B**