Homework riddle

I think all those on the summer holidays must be REALLY missing school, so here's a homework riddle to keep you in touch with your second homes, and also for those of us who can't remember what its like to be at school again!

A teacher at a local school tries to improve the young childrens arithmetic. He explains and demonstrates arithmetic using measuring instruments and objects. For homework he asks the children to use instruments and objects they may have at home to help with their homework. The next day he opens a book belonging to little Zakaria and was dismayed to see:

7 + 10 = 5

8 + 5 = 1

4 + 11 = 3

9 + 7 = 4

6 + 8 = 2

5 + 7 = 12

He imediately sends for little Zakaria and says, 'This work is a disgrace, only one right out of six simple additions, Little Zakaria protests that they are all the right answers, and when he explained why they were correct the teacher agreed with him that all the sums had the right answers.

WHAT DID ZAKARIA SAY TO THE TEACHER?

Did Zakariya use a clock?

eg. 7 plus 10 numbers going round the clock is = 5 etc.

Wasalaam

masha-Allah you are both correct, the young boy had used a 12-hr clock to measure time, i think muslimah_91 gave the biggest clue, this was a difficult one to figure out:

7 o'clock plus 10 hrs is 5 o'clock

8 o'clock plus 17 hrs is 1 o'clock

etc.