PSLE Bobby had red pencils and black pencils for sale. He sold 140 red pencils. 30% of the pencils sold were black.
- How many red pencils and black pencils did Bobby sell altogether?
- He sold 50% of his pencils. 80% of the pencils left unsold were red. How many black pencils did Bobby have at first?
| |
Red |
Black |
Total |
Total in percent |
| Before |
15 u |
5 u |
20 u |
100% |
| Change |
- 70%
- 7 u |
- 30%
- 3 u |
- 100%
- 10 u |
- 50% |
| After |
80%
8 u |
20%
2 u |
100%
10 u |
50%
|
30% =
30100 =
310 Total number of pencils sold = 10 u
Number of black pencils sold = 3 u
Number of red pencils sold
= 10 u - 3 u
= 7 u
7 u = 140
1 u = 140 ÷ 7 = 20
Total number of pencils sold
= 10 u
= 10 x 20
= 200
(b)
Percent of the pencils left unsold = 100% - 50% = 50% 50% of the pencils that were sold = 10 u 50% of the pencils that were left unsold = 10 u Percent of the pencils left unsold that were red = 80%
Percent of the pencils left unsold that were black = 100% - 80% = 20%
Number of the pencils left unsold that were black
= 20% x 10 u
=
20100 x 10 u
= 2 u
Number of black pencils that Bobby had at first
= 2 u + 3 u
= 5 u
= 5 x 20
= 100
Answer(s): (a) 200; (b) 100
