how can I find out what 100% of population is when I only know what 70% of population is?
You can make a ratio and cross-multiply.
For example, if I know that 70% of the population is 250, then I'd set up the ratio like this:
250/70 = x/100
Cross-multiplying means you'd multiply 250 by 100 and 70 by x
70x = 25000
Now divide both sides by 70, and you'll find out what x is.