One hundred cartoon ants are arranged on a parade ground in rows of ten. A watching ant does subtraction sums to work out how many ants are left after rows of ten leave the ground. The ant checks her calculation using the reverse operation by adding the rows to the remainder as they return. She visibly records and talks through her calculations as the rows come and go.
Before watching this clip, count forward and bacwards in tens. Write or display the 10s up to 100 on the board. Discuss the relationship between number bonds to 10 and number bonds to 100. Explain that if you know your number bonds to 10 then you know them to 100. Display 10 - 2 = 8 and 2 + 8 = 10. Then repeat with the related facts 100 - 20 = 80 and 20 + 80 = 100. Watch the clip then give the pupils some subtractions to carry out individually or in small groups.