This uses £1 (100's), 10pences (10's) and pennies (units) to show what is being done when exchanging. I called it subtraction by borrowing as that's what most teachers and pupils say. I prefer exchanging, as this is what is actually being done. Borrowing comes from the other method taught in the past, where the borrowed one is paid back.
This is a way to remember the exact trig values needed for higher GCSE using your left hand. There is a sheet to hand out that can be stuck in pupils’ exercise books as a reminder and a powerpoint explaining the method. It’s taken me a long time to write this so just trying to get some money for the time I put in. Thanks