15 electricians worked for 24 days to the whole job, now, there are 15 of them, so on any given day, each electrician worked one whole day, in 24 days, that one electrician worked 24 days total.
now, there were 15 electricians on any given day though, since each one of them worked the whole day that one day, so the "days work worth" on a day is 15, so the house gets 15days worth of work because of that.
so how many "days worth" did all 15 do on the 24 days, well, 15+15+15+15+15+15+15+15+15+15+15+15+15+15+15+15+15+15+15+15+15+15+15+15, namely 15 * 24, or 360 days worth of work.
since it takes 360 days worth of work to do the whole wiring, in how many days would 18 electricians do it? 360/18.