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
