Need help on this P6 Maths question.
A shopkeeper had some red and blue pens. The number of red pens
was 25% of the total number of pens. After selling away 30 red pens
and 30 blue pens, the number of red pens became 25% of the number
of blue pens left. How many pens did he have at first?
25% of all pens are red = 75% of all pens are blue
Ratio of red to blue = 25 : 75 = 1 : 3 = 3 : 9
After selling 30 red and 30 blues pens, amount of red pens = 25%
of blue pens,
hence, new ratio = 25 : 100 = 1 : 4 = 2 : 8
Since the same amount of red and blue pens are sold, you can
directly compare the original ratio of pens to the new ratio of
pens.
Original red to blue = 3 : 9, new red to blue = 2 : 8
Hence, you will realise that:
-Original total amount of pens = 12 units
-1 unit of pen = 30 pens
Hence, total amount of pens at first = 12 X 30 = 360 pens