$$3:7=15:x→\dfrac{3}{7}=\dfrac{15}{x}→3x=105→x=35$$ or $$3$$ multiply by $$5$$ equals $$15$$, so if we multiply $$5$$ by $$7$$, we get $$35$$. 
The ratio of your pears to oranges is $$3:7$$, and you have $$15$$ pears. How many oranges do you have? 

(a) $$7$$ 
(b) $$15$$ 
(c) $$21$$ 
(d) $$35$$ 