Counting by 10 and 100 in the service of calculating
Counting by 10 or 100 is only of value to calculating if each act of counting 10 more is taken to mean increasing the total by 10. This enables using counting by tens and units from the middle of the decade to find the sum or difference of two 2-digit numbers (see Aspect 4).
Children often learn to produce the number words corresponding to multiples of ten well before they learn that each sequential counting word (ten, twenty, thirty,…) also increases the total by ten.
Counting by 10 or 100