 I am Josh Sypes. I teach high school and middle school math at Oxford Junior Senior High School. And I will be just doing a short little lesson on how to effectively do mental math. A lot of mental math is an understanding of place value and the fact that the 7 is in the 1's place and the 6 is in the 10's place. So what this means is it's not just 6 and 7. It's 6 10's, which makes it 60 and 7. Likewise, this would be 90. So to do this effectively, mentally, what you can do is you can take 7 plus 4 is 11 and 60 plus 90 would be 150. So in your head, you can do 150 plus 11 would be 161. Using the fact that place value is important to when grouping and those types of things will help you effectively do mental math better.