John has a total of 45 of blue and black notebooks.
15 of the blue notebooks and half of the black notebooks are new. How many of his black notebooks are new if he has a total of 12 new notebooks?
| |
Blue notebooks |
Black notebooks |
Total |
| Total |
5 u |
2 p |
45 |
| New |
1 u |
1 p |
12 |
| Old |
4 u |
1 p |
33 |
Number of old notebooks
= 45 - 12
= 33
1 u + 1 p = 12 --- (1)
4 u + 1 p = 33 --- (2)
(2) - (1)
(4 u + 1 p) - (1 u + 1 p) = 33 - 12
4 u - 1 u = 21
3 u = 21
1 u = 21 ÷ 3 = 7
Number of new black notebooks
= 12 - 1 u
= 12 - 7
= 5
Answer(s): 5
