Mentally Add and Subtract 10 or 100

Here are some number lines that need to be completed. There are two number sequences, one to test addition skills and the other for subtraction. Both sequences include counting by 10, but the subtracting question has scholars count backwards, subtracting 10 each time. Because there is an explanation at the bottom of this page, consider projecting this as an all-class warm up.  Keep it covered up until after scholars have shared their responses. Depending on your pupils, the explanation may or may not be helpful.