4000-5-10-15-...-95-100=?

4000-5-10-15-...-95-100=?

First calculate the sum of 5 + 10 + 15 + 20 +. + 100 and subtract it from 4000
5+10+15+.+100
=5+2x5+3x5+...+20x5
=5(1+2+3+...+20)
=5x(21x10)
=1050
4000-1050=2950