how to get percentage of two numbers and calculate from a percentage

Question

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.

Solution 1

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;

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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

Solution 2

This is a practical explanation with words, since you're probably not into theory that much

Looking for a percentage

Write 400 out of 10'000 as a fraction, because that is what you look for: 400/10'000 = 0.04 in one (since 10'000 is all you got, hence one). That would mean 0.04*100 in 100, or 4%.

Equivalent statements:

    400/10000
    0.04
    4%

Any given number x in a population of y is (x/y)*100%.

Given a percentage

Note how the percent sign has two zeros in it, it literally means "per hundred". So if you do *100%, it's actually the same as multiplying by one, just another notation.

The other way around, again, think in words. Let's say 50 is 20% of someting. That means there is another 80% to the total; 50*(100/20) = 250.

Equivalent statements:

    50 = 20%*x
    50 = 0.2*x
    250 = x