Answer:
5 boxes of diaries
6 boxes of pencils
10 boxes of rulers
Explanation:
In order to find the smallest number of units that would be possible to buy, find the least common multiple between the number of units in each box:
![12\ \ 10\ \ 6\ |2\\6\ \ \ \ 5\ \ \ 3\ |2\\3\ \ \ \ 5\ \ \ 3\ |3\\1\ \ \ \ 5\ \ \ 1\ |5\\1\ \ \ \ 1\ \ \ 1\ | = 2*2*3*5=60](https://img.qammunity.org/2021/formulas/mathematics/college/k8zejgfbalmb78ef5pd0h5qnu8nrrpwwc0.png)
The number of boxes required for each item to get 60 units is:
![d=(60)/(12) = 5\ boxes \\p=(60)/(10) = 6\ boxes \\r=(60)/(6) = 10\ boxes](https://img.qammunity.org/2021/formulas/mathematics/college/c6oncfkif5rn0wnre9n4wq5h27iciu14wx.png)
the smallest number of boxes of each item he could buy is:
5 boxes of diaries
6 boxes of pencils
10 boxes of rulers