John has a total of 40 of black and purple notebooks.
13 of the black notebooks and half of the purple notebooks are new. How many of his purple notebooks are new if he has a total of 15 new notebooks?
| |
Black notebooks |
Purple notebooks |
Total |
| Total |
3 u |
2 p |
40 |
| New |
1 u |
1 p |
15 |
| Old |
2 u |
1 p |
25 |
Number of old notebooks
= 40 - 15
= 25
1 u + 1 p = 15 --- (1)
2 u + 1 p = 25 --- (2)
(2) - (1)
(2 u + 1 p) - (1 u + 1 p) = 25 - 15
2 u - 1 u = 10
1 u = 10
1 u = 10 ÷ 1 = 10
Number of new purple notebooks
= 15 - 1 u
= 15 - 10
= 5
Answer(s): 5
