Fred is 58 yes old, and irene is 18. how many years ago was fred 5 times as old as irene. algebra must be used

So let's take a peek at both's ages, keep in mind, every year, is 1year added to Irene and 1year added to Fred
so... if we look at their ages  

notice, Fred is always 40years older than Irene

thus, whatever age Irene is, let's say "i", then Fred is " i + 40 "

now, when is Fred 5 times Irene's age or 5*i or 5i? well, 

f = fred's age    i = irene's age

f = i + 40
now if  f = 5i

5i = i + 40   <---    solve for "i" to see how old Irene was then