Explanation:

Let’s break it down:

- Call the speed of the boat in still water “b” and the speed of the stream “s.”

- Downstream speed = (b + s), upstream speed = (b – s).

- First trip: 60 km downstream + 20 km upstream in 4 hours gives us:

60/(b+s) + 20/(b–s) = 4

- Second trip: 40 km downstream + 40 km upstream in 6 hours gives us:

40/(b+s) + 40/(b–s) = 6

If we solve (a bit of algebra):

Multiply the second equation by 1/2:

20/(b+s) + 20/(b–s) = 3

Subtract this from the first equation:

[60/(b+s) + 20/(b–s)] – [20/(b+s) + 20/(b–s)] = 4 – 3

So:

40/(b+s) = 1

Therefore, (b+s) = 40

Next, from the substituted equation:

20/40 + 20/(b–s) = 4

So, 0.5 + 20/(b–s) = 4

20/(b–s) = 3.5

(b–s) = 20/3.5 = 5.714

Now,

b + s = 40

b – s = 5.714

So,

2b = 45.714

b = 22.857

So s = 40 – 22.857 = 17.143 km/hr

Let’s compare with options:

Option 1: 24

Option 2: 16

Option 3: 18

Option 4: 20

The closest integer is actually 18. So, the answer is:

Option 3, 18 km/hr

- Option 1 (24): Way too high.

- Option 2 (16): Close, but not quite.

- Option 3 (18): Closest to exact calculation, makes sense.

- Option 4 (20): Still feels off, not matching.

What this really means is, check your math—18 is the speed of the stream.