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?
Originally posted by EK: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
Original r:b is 1:3
after selling 30red and 30blue, the ratio is 1:4.
We can tell that difference
1/3 x 1/4 = 30
total number of pens =12 x 30 = 360.
Draw model is a good way.
Thanks, got it.
Something I want to ask:
(7/8×23×24×25)÷13÷14=z
23×24×25=x
7/8×x=y
y÷13÷14=z
Is there any difference between both statements? My tuition teacher said the former was equivalent to no working as it was plucking numbers from the air. I thought the latter was the same as the former? I can use the former in my exams but it actually cannot be used during exams? Someone please reply.
Depend on the marking scheme.
Though both is the same.
Step by step to derive to the answer is the safer method.
Let's say quadratic formula. You have the equation, the values, and you just subt into the formula and wrote it on the paper. It's counted as correct. But it would be clearer if you could answer the question first by stating the formula. and show step by step (i.e inside sqrt, then +/- , then divide.)
Unless the question ask you to prove a certain value, I see no reason why it can't be use in your case. I feel that both is acceptable. Stricter teacher would penalize for not writing out the 'longer' way of deriving to the final answer.
Depend on what the question ask.