Carbon and Oxygen react together to produce CO₂ in the ratio of 3:8 by mass. What mass of carbon should be used to react completely with 20 g of oxygen?

The mass of carbon is 7.5g.
According to the question:- 
 $\underset{\mathrm{Carbon} }{C}+ \underset{\mathrm{Oxygen} }{O_{2 }} \to \underset{\mathrm{Carbon} \mathrm{Dioxide}}{C{O}_2 }$
 

Ratio of mass of Carbon and Oxygen in CO₂  =    $\frac{\mathrm{Mass} \mathrm{of} \mathrm{C}}{\mathrm{Mass} \mathrm{of} \mathrm{O}}$     $\Rightarrow \frac{3}{8}$    (given)
 

Let ‘x’ g of carbon is required to react completely with 20 g of oxygen. 
 

Now, according to the law of constant proportions 
$\frac{\mathrm{Mass} \mathrm{of} \mathrm{C}}{\mathrm{Mass} \mathrm{of} \mathrm{O}}=\frac{3}{8}$ 

The mass of carbon should be used to react completely with 20 g of oxygen

$\Rightarrow \frac{x}{20}=\frac{3}{8}$           

x =  $\frac{3\times 20}{8}$   

=  7.5g
So, the mass of carbon required = 7.5g