Problem Solving (PS) | If a stationery store owner buys 50% more identically-priced calendars

Let's say she buys A calendars for $B per calendar. She pays a total of $AB for A calendars.

Now if she buys 50% more calendars = 3A/2, she pays 20% less price per calendar = $4B/5. She pays a total of $3A/2 * 4B/5 = $6AB/5

The question says that the new amount ($6AB/5) = 120 times the price of each calendar = $120B

6AB/5 = 120B will give me value of A. A = 100.

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