break apart the addends to find the sum 18+21
we break apart the first numer:10+8the second:20+1and add like this: 18+21=10+8 + 20+1now, we can mix up the order: 18+21=10+8 + 20+1= 10+20 + 8+1add the parts:18+21=10+8 + 20+1= 10+20 + 8+1=30+9=39