Example on worksheet (subtracting by adding up)

50-29?

29+20=49

49+1=50

20+1=21

50-29=21

My question is where did the 20 come from? can you help me understand this?

(49 - 29) + 1 =(50 - 29)

Step-by-step explanation:

The 20 comes from the subtraction of 29 in both sides of the previous step equality. In the following, I transcript the complete procedure and I add the step that you need to understand why 20 appears (in bold numbers):

29+20=49

49+1=50

(49 - 29) + 1 =(50 - 29)

20+1=21

50-29=21

hence, it was only nesseraty to subtract 29