(i) Chalk contains calcium, carbon and oxygen in the ratio 10:3:12. Find the percentage of carbon in chalk. 

(ii) If in a stick of chalk, carbon is 3g, what is the weight of the chalk stick?

(i) Let us given that,  The ratio of calcium, carbon and oxygen in the ratio 10 : 3 : 12

Sum of all parts = 10 $+$ 3 $+$ 12 = 25

Carbon percentage = (parts of carbon/ Sum of all parts)  $\times$ 100

= (3/25)  $\times$ 100

= 12%

(ii) Given that,  In a stick of chalk, carbon is 3 g,

We found that,  Carbon percentage in chalk = 12%

which shows that 12% carbon = 3 g

Consider that weight of the chalk is y 

Thus, 12/100 $\times$ y = 3

Weight (y) = 3 $\times$ 100/12 = 25 g.