What was the challenge given?
Just keep up adding the given numbers and remember only the last two digits of the sum.
The sum of all numbers from 1 to 100 is 5050, so if you know the sum of all the listed numbers, you will know the missing number as well.
At the end, if the result is less than 50, subtract it from 50. If the result is larger than 50, subtract it from 150.