Can you arrange four 9's and use at most 2 math symbols, to make the total 100?

Prove that twice of ten is the same as twice of eleven.

Can you replace the X (both small and big x) with the numbers between 1 to 19 such that all the rows of 3 numbers between X (big x) sum to 23?

John was working on a Chemical Mixture whose weight comprise 90% liquid and 10% solid. The total weight of the mixture is 20 pounds. After a while, John noticed that some of the liquid evaporated and now the liquid comprises just 50% of the weight.

What is the weight of the chemical mixture now?

Can you find a pattern and the missing number in the series given below?

100, 155, 258,? , 584, 819.

What is the fastest way to double your money?

There was an aeroplane crash, every single person died, but two people survived. How is this possible?

By using all numbers, i.e. 123456789 and subtraction/addition, operators number 100 can be formed in many ways.
Example: 98 + 7 + 6 - 5 - 4 - 3 + 2 - 1 = 100

But if we add a condition use of the number 32 is a must. Then there are limited solutions.
One of such solution is: 9 - 8 + 76 + 54 - 32 + 1 = 100

Can you tell me any other solution?

2, 3, 5, 9, 17, _ What is the next number in the sequence?

I am the one who remains awake when you sleep. But I need to rest when you awake. Who Am I?