It takes 15 electricians 24 days to wire a new housing subdivision. How many days would 18 electricians take to do the same job, assuming all electricians work at the same pace?

So to set up this equation, you have to do
Time1*Workers1=Time2*Workers2
In this case, Time1=24 days, Workers1=15 electricians, and Workers2=18 electricians, and we have to solve for Time2

Thus we get 15*24=18*Time2
Time2=15*24/18=20 days