This is a job for a computer.
It does it by brute force. Here are the steps.
for i = 201 to 1000
check if i is even by doing i mod 2 = 0 then exit this routine. that is if 0 shows up when you do a mod operation on it then leave. The number you are testing is even. Even numbers except 2 are not prime.
next find the square root of i. divide by all the odd number up to that square root.
for j = 3 to sqrt(i) step 2
if i mod j = 0 then exit this subroutine
if you find nothing that divides into i then record the number as prime.j
Try an example
201 is not prime it is divisible by 3
205 is not prime it is divisible by 5
207 is not prime it is divisible by 3
209 well let's see what happens to 209
the square root of 209 is 14
209 is not divisible by 3
209 is not divisible by 5
209 ÷ 7 = 29.85 so 7 does not go into 209 evenly
209 is not divisible by 9. If 3 doesn't work, 9 won't either.
209 ÷ 11 = 19 wwwooops. This number is not prime. 11 divides into it equally.
This method works extremely well for small numbers like 201 to 1000. It is hopeless for much larger numbers.
