1) 12 = 10+2
2) 12+7 = (10+2)+7 = 10+(2+7) = 10+9 = 19
6+5 = (5+1)+5 = (5+5)+1 = 10+1 = 11
3) 13+6 = (10+3)+6 = 10+(3+6) = 10+9 = 19
15+3 = (10+5)+3 = 10+(5+3) = 10+8 = 18
1) 12 = 10+2
2) 12+7 = (10+2)+7 = 10+(2+7) = 10+9 = 19
6+5 = (5+1)+5 = (5+5)+1 = 10+1 = 11
3) 13+6 = (10+3)+6 = 10+(3+6) = 10+9 = 19
15+3 = (10+5)+3 = 10+(5+3) = 10+8 = 18