I think the best way in memorizing the roots of numbers 1 - 30 is by through methods that would suitable for you but you must consider is the pattern in each number or the square, every number has it pattern and i know you realized it by now. I hope this would help
So it's multiple choice?
whichever looks most like this is the answer:
30mn+6n-36m
The answer is 68
hope this helps
