Question

Roy used 1/4 of his money on 3 pens and 6 notebooks. The cost of each pen is 3 times the cost of each notebook. He bought some more pens with 2/3 of his remaining money. How many pens did Roy buy altogether? ​

Answer 1

Answer: 13 pens

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Step-by-step explanation:

Roy spends 1/4 of his money on 3 pens and 6 notebooks. That means 3/4 of it is leftover.

2/3 of 3/4 = (2/3)*(3/4) = 1/2 of his money is spent on buying some unknown number of additional pens. We'll come back to this later.

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In terms of cost,

1 pen = 3 notebooks

which is another way of saying 1 pen is the same price as 3 notebooks.

Multiply both sides by 2 to get

2 pens = 6 notebooks

Therefore, saying "3 pens + 6 notebooks" is the same as "3 pens + 2 pens = 5 pens" when just thinking about costs.

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In short, Roy buying 3 pens and 6 notebooks is the same as him buying 5 pens. He spends 1/4 = 25% of his money on getting these 5 pens.

Multiply those two values by 2 to find that 50% = 1/2 of his money would allow him to get 10 pens.

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Recall that at the end of the first section, we concluded that Roy spent 1/2 of his money on buying those unknown additional number of pens. Then the previous section mentioned that 1/2 of his money gets him 10 pens. Therefore, he must have bought 10 additional pens on top of the original 3 mentioned in the instructions.

Overall, he purchased 3+10 = 13 pens

Answer 2

13 pens

Step-by-step explanation:

If a pen costs 3 times the cost of a notebook, then the cost of 3 notebooks will equal the cost of a pen. The 6 notebooks that Roy bought are equivalent in cost to 2 pens, so 1/4 of Roy's money is the cost of 3+2 = 5 pens.

After the first purchase, Roy has (1 -1/4) = 3/4 of his money remaining. If he spends 2/3 of that on more pens, he will have spent ...

(2/3)(3/4) = 2/4

of his money on more pens. We've already seen that 1/4 of his money buys 5 pens, so 2/4 will buy 10 more pens.

Roy bought 3 +10 = 13 pens altogether.