math ASAP
posted by eduardo .
please explain how to use doubles plus 1 and doubles minus 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=61=5
7+8=161=15
11+10=111=21
11+10=20+1=21 
0+1=1