15 pencils were left from the box of pencils Amaiya bought.
Step-by-step explanation:
Number of pencils in box = 36
Ratio of used pencils to not used = 7:5
Let,
7x be the total pencils used.
5x be the total pencils not used.
Therefore,
7x+5x=36

Dividing both sides by 12;

Therefore,
Used pencils = 7x = 7*3 = 21
Not used pencils = 5x = 5*3 = 15
15 pencils were left from the box of pencils Amaiya bought.
Keywords: ratio, multiplication
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