The unit digit of 1! + 2! + 3! + ... + 99!

This is a good question for many competitive exams which may seem easy at first but if you do not know the correct approach for this then you may not be able to solve it. This question can also come in maths olympiads as well.

This is an important topic for class 11 and class 12. This lecture is also sufficient for any competitive exam or any maths olympiad.

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The problem discussed in the video: Find the unit digit of 1! + 2! + .. + 99!

The solution to the problem: Follow the following steps,

1. Find the unit digit of all the factorials from 1 to 99
2. Unit digit of all the factorials are zero after 4
3. Find the sum of unit digit of all the factorials from 1 to 4
4. Find the unit digit of the sum, which is the answer
5. You will get the final answer as 3

Extension Problem: Find the tens digit of - 1! + 2! +.... + 99!



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