Here’s a quick trick for mentally multiplying two numbers which are greater than or equal to 10 but less than 20.

Let’s use 18 and 19 as an example.

Add the second digit in one of the numbers to the other number.
18 + 9 = 27
Now, add a zero (mulitply by 10)
27 x 10 = 270
Multiply the last digit of each number
8 x 9 = 72
Add the two values together
270 + 72 = 342

18 x 19 = 342

Math math trick, mental math