Here's a brief Wikipedia explanation.
You can think of doing modular artithmetic kind of the same way you would, say, figure time on a clock.
In modulo 12, 10+3 = 1
because 10+3=13, of course, but 13 mod 12 = 1 because 13 divided by 12 leaves a remainder of 1.
Similarly, on a clock, 10pm + 3 hours = 1 am, not 13pm.
And then 1:00 + 18 hours = 1:00 + 6 hours = 7:00.
Because 18 mod 12 = 6.
So you've been using modular arithmetic as long as you've been telling time without knowing it was modular arithmetic. That knowledge will hopefully make you more comfortable with using it now in other modulos than 12.
Year 7 means 11 years old if he is from England so would probably not be familiar with that notation. However it would not be part of the syllabus either so you never know.
If I'm right he need a different solution to his previous post