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Irene bought 10 pens with 2/5 of her money. If she bought another 24 pens then she would need $18 more.
What is the amount of money she had left after buying the 10 pens.
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Such problems on "shortage the money" are traditionally considered
as entertainment Math problems.
Therefore, I will give you MENTAL solution below.
Since Irene bought 10 pens with 2/5 of her money, it means that
she could buy 15 more pens for her remaining 3/5 of money.
But the problem said that if she bought additional 24 pens (additional to that 10 pens),
she would be $18 short.
It means that 24-15 = 9 extra pens cost $18. Hence, each pen costs 18/9 = 2 dollars.
Great (!) Half of the problem is solved. The rest is just easy.
One pen costs 2 dollars; hence, 15 pens cost 30 dollars,
and it is the amount of money she had left after buying the 10 pens.
ANSWER. After buying 10 pens, Irene had left 30 dollars.
Solved.