In Japanese, the words 13 and 30 are identical, and the meaning must be gained through context. The same holds true for the pairs 14 and 40, 15/50, 16/60, 17/70, 18/80, and 19/90. This is one of the reasons Japanese are so good at math.
When speaking English, they often confuse the pairs, as they cannot properly be represented and thus become mixed up.
To practice differentiating these numbers, bring one hundred pennies into the classroom, and a noisemaker. The noisemaker can be in the form of a bell, a bike horn, a musical instrument, or anything able to produce noise. Have the student begin counting the pennies. When the student reaches one of the difficult word pairs, create a noise and say the number. Have the student repeat the number. Repeat the activity several times until the student is able to count to 100 unassisted.
Next, practice speaking by writing the word pairs on a whiteboard or sheet of paper. Point to them and have the student recite the word. This can easily be turned into a game for groups, where each correct answer gets one point, and each incorrect answer gets 0 points. All teams play to 100.