"1" divides the work that 65 people need to do on 18 into two parts: 60 people do it on 18 and 5 people do it on 18; Divide the work that 60 people need to do in 20 days into two parts: 60 people 18 days and 60 people do it in 2 days;

"2" means that "5 people do 18 days of work" is as much as "60 people do 2 days of work".

"3" means that "five people work nine days" is as much as "60 people work 1 day".

The more people there are, the lower the work efficiency (that is, the amount of work that everyone can complete every day).

"5" is because it takes 60 people 20 days to complete the work of "controlling the epidemic".

So: the total workload of "the work of controlling epidemic situation" is equal to 60 * 20 * (the work efficiency when 60 people work)? Namely:1200 * (working efficiency when 60 people work)

So: the total workload of "the work of controlling epidemic situation" is equal to 65 *18 * (the efficiency of 65 people's work)? Namely:1170 * (efficiency when 65 people work)

"6" is because:1200 * (working efficiency when 60 people work) is equal to1170 * (working efficiency when 65 people work).

So: (Efficiency when 60 people work): (Efficiency when 65 people work) =1170:1200? Namely: 39: 40