After learning 1-12, Newcomers are ready for the teens. These can be taught as 13 = 3 + 10 = thir(3) + teen(10), etc. Teen stands for ten. After students learn these, I introduce the tens. 20 = twen(2) + ty(10), etc. These words use -ty to indicate x10.
Next, I teach word stress. Teens stress the second syllable and tens stress the first syllable - thir TEEN, THIR ty. This is very difficult for students to hear and say. I raise my hand to indicate the stressed syllable. After lots of practice, we do dictation pairs of teens and tens (14 or 40, 15 or 50, etc.)