C# – how to get percentage of two numbers and calculate from a percentage

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I am developing a application that need to calculate percentage from two numbers. dotnet have some method for do this? i like to write this calculate by myself. but i don't good in mathematics unfortunately.
really, how to get percentage in mathematics? for example i have 10000 request in my application, and 400 request of them are for special part of my application. how to calculate what percent of request are request for a part of my application?
and how to calculate reverse, for example, i know 5 percent of users like my special post, how can calculate number of users that like that post?
Thank you and sorry for my bad English.

Best Answer

dotnet have some method for do this?

Since it's just arithmetic, every programming language has this. Generally, the calculation would be...

If you want to find "What percent of Y is X?", such as "What percent of 100 is 50?", then you divide X by Y:

var percent = x / y;

or:

50 / 100 = 0.5

Note, however, that in programming languages you do have to be concerned with data types. In C# for example, if X and Y are ints then the result will be 0, not 0.5. This is because an int divided by an int is also an int.

In that case you'd need to cast them to something like a double to get sub-integer precision:

var percent = (double)x / (double)y;

how can calculate number of users that like that post?

That would just be the reverse. Consider the values that you do know:

I have 100 users.
0.05 of the total like the post.

Then the number of users who like the post is the percentage times the total:

var numberOfLikes = percentOfLikes * totalUsers;

or:

0.05 * 100 = 5

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