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