Place Value and Reversibility

I had 64 apple I

Place Value

64-36 ?

60+4-30+6

60-30=30

30- 6=24+4=28 if you want to check your answer you go  28 +36=64

Reversibility

137-56 ?

56+___ =137

56+4=60+40=100+37=137  

4 + 40 + 37=81+56=137