I just started school last tuesday, and in algebra 1 were reviewing a lot of things we should've learned last year. And supplementary numbers are something im terrible at. I can never understand them. My teacher wants us to put it in like an "let x=" equation thing. I can find the answer but im not sure how to put it into an equation like example: 4(12)=90x or something. and in the end it says x= the answer. and if you plug x into the equation it should equal or something. and my problem is..

"Separate 90 into two parts so that one part if four times the other part."

can anybody help elaborate on that? or tell me how to set it up?