Mrs Koh bought some pens, files and erasers. The ratio of the number of pens to the number of files to the number of erasers she bought was 1 : 2 : 3. The cost of each pen and eraser is $2.50 and $0.50 respectively. If she spent $60 on the pens and the erasers, how many files did she buy?

Answer 1

Say she bought x pens, 2x files, and 3x erasers. Adding up the cost of the pens and erasers, we have

2.50x + 0.50(3x) = 60.00
x = 15

So, she bought 30 files