Out of the students at a school 55% are female, there are 900 males , how many students are there?

2,000

55% is just 50%+5%
The remainder is 45%, which is 50%-5%

Since we know that the difference is a moved 5%, let's divide the 900 males by 9, to bring that 45% down to 5% chunks, because 5x9=45. A such, 900/9=100.

Now we have a base number. Every 5% is 100 people. If 900 is 45%, adding 2 more 5% chunks (100+100) gives you the 55% female populace, 1,100.

Add the two together and you get 2,000 total students.

To check your work, you reverse the process.

50% of 2,000 is 1,000
You then take 5% of 2,000 (100) off of 1,000 to get 900 (45% Males)
Take that 100 (5%) and add it to the other 1,000 to get 1,100 (55% Females)

Since we have our original exact male populace of 900, and the female populace matches the 55%, you know you've done it correctly.