Why 1729 is a magic number? Answer Logically.

Replace the question mark with the number below given a simple math problem.

A famous swimmer can swim downstream in a lake in exactly 40 minutes with the lake current.
He can swim upstream in that lake in exactly 60 minutes against the lake current.
The length of the lake is 2 km.

How long he can cover the distance of one side at a still lake with no current?

Place the numbers from 1 to 9 in the circles in a manner that each side of the triangle sums up to 17.

If "P" means "-", "Q" means "/", "R" means "+", and "S" means "*" then find the value of the following:

21 Q 7 R 9 S 10 P 13

A man is surviving on an island along with his friend. They have been starving for four days for food. Suddenly, they find a fisherman. The friend goes with the fisherman in order to catch the fish (if they can find any).

The fisherman returns after some time but he is alone and has some salmon that he had prepared. When asked, the fisherman tells him that his best friend fell off the boat and drowned in the water. Starving for so long, he eats the meal while crying for his friend.

After being rescued, he goes to a diner and orders salmon. When he eats his meal, he jumps and commits suicide. Why did he do it?

Find The Next Number In The Sequence

30 10 15 13 0 16 -15 ?

I possess a head

I flaunt a tail

But I don't have mouth or legs

Can you find out who I am?

Can you find out the missing number?

? 4 5 3 5

9 5 7 10 3

Many have heard it, but nobody has ever seen it, and it will not speak back until spoken to. What is it?