Ola Michael

media das classes

(1000 + 600)/2 = 800

(1400 + 1000)/2 = 1200

(1800 + 1400)/2 = 1600

(2200 + 1800)/2 = 2000

soma

S = 18*800 + 10*1200 + 8*1600 + 4*2000 = 47200

numero de pessoas

n = 18 + 10 + 8 + 4 = 40

media

M = S/n = 47200/40 = 1180 reais

pronto