The square root of number 121 is "11". What is the square root of the number "12345678987654321."?

How many triangles are there on the puzzle below?

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

Replace each alphabet with the number (1-9) to make the below equation correct.

AB * C = DE - F = GH / I

If we add four times the age of John four years from now to five times his age five years from now we get ten times his current age.

How old will John be two years from now?

Find three whole, positive numbers that have the same answer when multiplied together as when added together.

Double it and multiply it by 4. Then divide it by 8 and you’ll have it once more. What number is it?

Get total of 120 by using five zeros 0,0,0,0,0 and any one mathematical operator.
Do you ?

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?

A 3 digit number is such that it's unit digit is equal to the product of the other two digits which are prime. Also, the difference between it's reverse and itself is 396.

What is the sum of the three digits?