Count, represent and write teen numbers. Start with 11 and 12, then proceed to 13 and 14, and so on until 19 and 20. See teen numbers as 10 and some 1s. Here's an example problem:

If we start with \(10\) pennies, how many more do we need to get to \(13?\)

Shown two ten frames, one fully filled and the other partially filled, count on to find the teen number. Solve one more and one less problems involving teen numbers. This skill can be practiced here. Answer missing addend problems for teen numbers. Write in missing numbers within 20. For example, 1, 2, 3, and so on, but 4, 7, and 18 are missing. Khan Academy covers all of this, except it doesn't use ten frames and doesn't cover writing teen numbers. For writing teen numbers, I suggest JoAnn's School.

Where is writing numbers 11-20 in the Common Core?

Watch these Khan Academy videos:

Teen numbers by Khan Academy

Conclude by giving your students this challenge:

Butterfly Flowers by NRICH

K.NBT.A.1: Compose and decompose numbers from 11 to 19 into ten ones and some further ones, e.g., by using objects or drawings, and record each composition or decomposition by a drawing or equation (such as 18 = 10 + 8); understand that these numbers are composed of ten ones and one, two, three, four, five, six, seven, eight, or nine ones.