13 x 7 = 28
Student's  page

Click on image above
 Watch Abbott and Costello do some mathematical hocus-pocus with multiplying 13 x 7 and getting an answer of 28.

Explain how Costello cheated with math to get that answer.

If Youtube version is not available, try this link.

Costello's Method
13

x7

---

21

 7

---

28
Explanation
Source: "Best of Abbott & Costello – Live”
Teacher's page