Question

Twenty litres of milk with unknown butter-fat content from Vat A is mixed with 10 litres of milk with 3% butter-fat from Vat B to produce milk with 3 43 % butter-fat content. Find the percentage of butter-fat in Vat A.

Answer 1

make sure the fat content of the parts adds up to that of the mix.
20x + 10*.03 = 30*0.343
Now finish it off.