Total people $=2u+10=40$  $2u=40-10=30$  Girls $=1u=30\div2=15$  Boys $=1u+10=15+10=25$  Girls $:$ Boys $\to$ $15:25$ $\to$ $3:5$ 
In a class of $$40$$ pupils, there are $$10$$ more boys than girls.  What is the ratio of the number of girls to the number of boys? 
Options:
(a) $$5:3$$ 
(b) $$3:1$$ 
(c) $$1:3$$ 
(d) $$3:5$$ 