Borrowing numbers

Today on Tuesday the 26th of August 45J learnt how to borrow numbers. This is a borrowing sum. 440 take 362 = 078. If theres a 0 in the sum its 10. Before you do borrowing, you have to know 3 digit take away sums. When the teacher said " Or right. Lets get harder". and wrote a sum up on the board. I went "ah! I know this! because I had learnt it with my tudor,Clair.