CCSS.MATH.CONTENT.1.NBT.B.2

Understand that the two digits of a two-digit number represent amounts of tens and ones. Understand the following as special cases: 10 can be thought of as a bundle of ten ones called a “ten”; numbers 11-19 are composed of a ten and additional ones; and multiples of ten from 10-90 represent one to nine tens and zero ones.