**Middle School Number Sense Lesson 18: Multiplying Two 2-digit Numbers whose Tens Digits Are the Same, and whose Units Digits Add up to 10**

Like the concept from last Tuesday, this idea is much simpler than its title suggests. It showed up

**12 times**this year--10 of those times at

**question # 13**. Before solving these problems, make sure the following conditions are met:

- Both numbers are 2-digit integers.
- The tens digits are the same.
- The units digits add up to 10.

**Number Dojo Level: 73**

This concept is SquareEnd5's first cousin. If you don't know yet how to square numbers ending in 5, please review that now so you can see the pattern of operations involved.

**How to solve:**

- Multiply the units digits and write this product down as the last 2 digits of your answer. (If the product is 9, write down 09 instead of 9).
- Multiply the common tens digit by its upper neighbor (itself plus 1). Write this down in front of what you wrote in step 1. You're done!

**Example 1: 42 x 48**

- Multiply the units digits. 2 x 8 =
**16**; write this down. - Multiply the common tens digit (4) by its upper neighbor (5). 4 x 5 =
**20**; write this in front. The answer is**2016**.

**Example 2: 29 x 21**

- Multiply the units digits. 9 x 1 = 9; write it as
**09**. - Multiply the common tens digit (2) by its upper neighbor (3). 2 x 3 =
**6**; write this in front. The answer is**609**.

**Example 3: 94 x 96**

- Multiply the units digits. 4 x 6 =
**24**; write this down. - Multiply the tens digit by its upper neighbor. 9 x 10 =
**90**; write this in front. The answer is**9024**.

**Example 4: 6.7 x 6.3**

- Don't let the decimals fool you. 7 x 3 =
**21**; write this down. - Multiply 6 x 7 to get
**42**and write this in front. Remember that there will be 2 decimal places, so the answer is**42.21**.

**Example 5: 198 x 192**

- This will work with 3-digit numbers, as long as the first 2 digits are the same in both numbers. Multiply the units digits. 8 x 2 =
**16**; write this down. - Multiply 19 x 20 to get
**380**; write this in front. The answer is**38016**.

**Here's a free worksheet to help you practice Mult10s,U+10:**

mult10su_10.pdf |

**And a video.**

Up Next for Middle School: MultDistrib2

Up Next for Middle School: MultDistrib2