The mean attendance of a school for the first 4 days of the week was 780 and for the

first five days it was 840.How many were present on Friday?

Pls answer it very quickly.

Let the first five days  be M, T, W, Th and  F.

For the first 4 days, mean attendance is 780, therefore we have:

(M + T + W + Th )/4 = 780 

(M + T + W + Th ) = 3120  ……(i)

For the first 5 days, mean attendance is 840, therefore we have:

 (M + T + W + Th + F )/5 = 840 

 (M + T + W + Th + F) = 4200  ……..(ii)

By subtracting (i) from (ii), we get

F = 1080.

Therefore, the number of students present on Friday is 1080.