Correct Answer - Option 1 : 30 km

**Given:**

Speed of boat in still water = 8 km/hr

Speed of stream = 2 km/hr

Difference of time taken during upstream and downstream = 2 hours

**Concept used:**

Downstream speed (S_{D}) = Speed of boat + Speed of stream

Upstream speed (S_{U}) = Speed of boat – Speed of stream

T_{D} = D_{D}/S_{D}

T_{U} = D_{U}/S_{U}

**Calculation:**

Let the distance travelled during upstream be D km

Distance travelled during downstream = D km

S_{D} = 8 + 2 = 10 km/hr

S_{U} = 8 – 2 = 6 km/hr

T_{D} = D/SD

⇒ TD = D/10

T_{U} = D/SU

⇒ T_{U} = D/6

According to question

T_{U} – T_{D} = 2

⇒ D/6 – D/10 = 2

⇒ 2D/30 = 2

⇒ D = 30 km

**∴ Distance travelled during downstream is 30 km**