**Solution :**

**Part filled by pipe A in 1 minute = 1/60**

**Part filled by pipe B in 1 minute = 1/40**

**Part filled by both pipes A and pipe B in 1 minute = 1/60+1/40 = (2+3)/120 = 5/120 = 1/24
**

Suppose the tank is filled in x minutes

Then, To fill the tanker from empty state, B is used for x/2 minutes and

A and B is used for the rest x/2 minutes

**x/2×1/40+x/2×1/24=1**

**⇒x/2[1/40+1/24]=1**

**⇒x/2×8/120=1**

**⇒x/2×1/15=1**

**x = 15×2 = 30 minutes**