Once, you break your ten into ones then instead of drawing the lines and crossing them you can use your fingers. Example If we have to subtract 18 cars from 20 cars. Then in one’s column, 0 – 8 does not make any sense, so we must regroup one of the two tens such the 0 now...
2+5=7 7−5=2 7=4+3 7−3=4 3+4=7 7−4=3 7+3=10 15+5=20 20=11+99+11=20 20−9=1120−11=9 (10+1+9=10+10=20) 20=12+88+12=20 20−8=1220−12=8 (10+2+8=10+10=20) 20=13+77+13=20 20−7=1320−13=7 ...