please explain how to use doubles plus 1 and doubles minus 1

Answer 1

This is a method for early learners to use to add.

when adding two consecutive numbers, double the lower number, add 1

2+3=4+1=5
7+8=14+1=15

or, double the higher number, and subtract one.
3+2=6-1=5
7+8=16-1=15

11+10=11-1=21
11+10=20+1=21

Answer 2

0+1=1