? (30*0.25 + x)/ (30+x) =0.40; thence 7.5 +x =12 +0.4x, hence x=7.5 kg;
.25(30)=x =7.5 and .4(30+y)=x+y 1) substitute x=7.5 into 2nd equation .4(30+y)=7.5+y 2) 12+.4y=7.5+y 3) 12.5-7.5=y-.4y 4) 4.5=.6y 5) (4.5)/.6=y 6) y= 7.5kg of cement added (37.5kg cement MIX) To check answer: original 7.5kg of pure cement(25% of 30kg) plus the added 7.5kg of pure cement = 15kg of pure cement. 30kg total mix plus added 7.5kg of pure cement = 37.5kg of cement mix. (15kg of pure cement)/(37.5kg of cement mix) = .4 or 40%
