8. Maggie had a total of 120 red and blue pens in the ratio 3 : 5. After she had given away an equal number of each type of pens, the number of red and blue pens left was in the ratio 3 : 8. How many pens did she give away altogether?
The differences in the number of red and blue pens before and after are the same – Constant Difference. So we need to make the differences in the two given ratios the same (by multiply them by the respective difference of the other ratio).
3 : 5 = 15 : 25
3 : 8 = 6 : 16
Total number of units (15 + 25). If you like you may show how you get 40.
40 u -- > 120
1 u -- > 120 ÷ 40 = 3
Units given away is (15 - 6) and (25 – 16). So total number is 18 units. Again if you like you may show how you get 18.
18 u -- > 9 x 3 = 54 pens
She gave away 54 pens altogether.
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