Which of the following statement is correct?

I. If downstream and upstream of a boat are 12 km/hr and 6 km/hr respectively, then what is the speed of boat in still water is 9 km/hr.

II. Length of train is 1200 meters. If it can cross a stationary pole in 10 seconds, the speed of train of train is 43.2 km/hr.

This question was previously asked in

DSSSB PGT Computer Science Female General Section - 11 July 2021 Shift 1

Option 1 : Only I

**Given**:

For statement (1)

Speed of boat in downstream (S_{d}) = 12km/hr

Speed of boat in upstream (S_{u}) = 6 km/hr

For statement (II)

Length of train (l) = 1200 m

It can cross the stationary pole (t) = 10 seconds

The speed of train is 43.2 km/hr

**Formula** **used**:

Downstream speed = speed of boat in still water + speed of stream

Upstream speed = speed of boat in still water - speed of stream

Distance = speed × time

**Calculation**:

For statement (I)

Let us consider the speed of boat in still water be x km/ hr and speed of stream = y km/ hr

Downstream = x + y

12 = x + y ----(1)

Similarly,

Upstream = x - y

6 = x - y

Solving equation (1) and equation (2):

⇒ x + x = 12 + 6

⇒ 2x = 18

⇒ 9 km /hr

Putting the value of x in equation (1) the we get the value of y,

12 = 9 + y

⇒ y = 3 km/hr

Means, boat speed in still water = 9 km/hr

**Hence**, statement (1) is correct

For statement (II)

Speed = (1200/10) = 120 m/S

Speed = 432 km/hr

**Hence** statement (II) is incorrect.