Number and Operations in Base Ten
Understand place value.
Understand that the two digits of a two-digit number represent amounts of tens and ones. Understand the following as special cases: [1-NBT2]
10 can be thought of as a bundle of ten ones, called a “ten.” [1-NBT2a]
The numbers from 11 to 19 are composed of a ten and one, two, three, four, five, six, seven, eight, or nine ones. [1-NBT2b]
The numbers 10, 20, 30, 40, 50, 60, 70, 80, 90 refer to one, two, three, four, five, six, seven, eight, or nine tens (and 0 ones). [1-NBT2c]