7+20+3+40= (7+3) + (20+40)= 10+60=70
60+1+20+ 9 = (60+20)+(1+9) = 80+10=90
40+5+25+10 = (40+10) + (5+25) = 50+30=80
15+7+5+13= (15+5)+(7+13) = 20+20=40