Example, 20 men are to finish a job in 4 days. how many mem are needed to finish it in 2 days? 

first, make a table.see picture one.

then, write it in rational formsee picture 2

then, inverse the ratio without the other quantity.see picture 3

do the same for direct proportion

the answer is 
it would require 40 men to do the job in 2 days
1 3 1