Let the speed of the stream is x km/hour. 

The speed of the boat in still water is 4 km/hour. 

The upstream speed of the boat is (4 – x) km/hour. 

The downstream speed of the boat is (4 + x) km/hour. 

Distance = 30 km 

We know that,

Time = \(\frac{Distance}{speed}\).

Now,

Time taken by the boat in going 30 km upstream = \(\frac{30}{4-x}\) hours.

And,

Time taken by the boat in going 30 km downstream = \(\frac{30}{4+x}\) hours.

Now,

According to given conditions

3(\(\frac{30}{4+x}\)) = \(\frac{30}{4-x}\)

⇒ \(\frac{3}{4+x}\) = \(\frac{1}{4-x}\)

⇒ 4 + x = 12 − 3x 

⇒ 4x = 12 − 4 = 8 

⇒ x = 2.

Hence, 

The speed of the stream is 2 km/hour.

(ii) Let Jayant answered x number of questions correctly and y number of questions he gives wrong answer. 

∵ Jayant answered 120 questions. 

∴ x + y = 120 … (1) 

∵ Jayant got 348 marks. 

∴ 5 – 2y = 348 … (2) 

(∵ Each correct answer awarded as 5 marks but wrong answer deducted 2 marks) 

Now,

Putting y = 120 – x from equation (1) in to equation (2), we get 

5x – 2 (120 – x) = 348 

⇒ 5x – 240 + 2x = 348 

⇒ 7x = 348 + 240 = 588 

⇒ x = \(\frac{588}{7}\) 

= 84.

Hence,

Jayant answered 84 number of questions correctly.