How I check my answer ?

Well.
That is quite challenging to do mathematics questions.
There  might be many solutions for one one mathematic question.
Sometimes, you may need more time to study the question to get optimal solutions.
However it might quite frustating if you do not know whether that is correct answer or solutions after you trying the questions for more than half an hour.

To ensure the "accuracy", I will normally checking my answer using computer.
It is not encouraging to do exhaustive method to do checking on hand, but we might use it by the help of computer.

Recently, I find out it is quite challenging to solve question regarding of disibility question.
You may check on the question in my other posts.
Extra questions: How many positive integers not exceeding 2001 are multiples of 3 or 4 but not 5?
Answer for revision book 5

I would like to introduce another method if you wish to check how accurate is your answer.
I will do some coding, in language C++ for this question.
I know you may ask what is C++ ? It is a programming language you may use to do a program.
Simply you may find some C++ online compiler and paste the code and run it.

Steps:
1. Please download the code file. If you cant view the file, you may right click the file, and click on open with then choose notepad to open the code
    Divisible question code
2. Please click on this link to open the online compiler
    C++ online compiler i am using
3. Copy and paste the code in C++ online compiler. Then click run.

You may get your answer.

You may modify the code when you have to check your answer for similar question but different value.

Have fun!