If the substance is 35.7 % C by mass, then in 100 g of substance there is 35.7 g of C. From the whole substance only 35.7 % is composed of C. Therefore to find the mass of C that can be recovered from 769 g of the substance, we need to calculate 35.7 % of 769 g. In 100 g substance - 35.7 g of C in 769 g substance - 35.7 /100 * 769 = 274.5 g of C can be recovered
