Back, Back, Back It Up — Math Tricks

My students that come to me for math on Mondays, usually understand the material by the time they take their tests.  However, they often make simple mistakes which lowers their grades.

With multiplying double digit numbers, they had 3 common mistakes.

The first was carrying the wrong number.

PPT’s Solution:   The number in the back of the line goes on the bottom.  So if they multiply 29x32, and they start with 2x9= 18, the 8 is in the back of the line and it goes on the bottom and they carry the 1.

The second was adding a number that they carried in twice.   This would happen, for example with the above problem, when they multiplied 3x2, they would add in the 1 from the 18 they had when they multiplied the one’s place numbers.

PPT’s Solution:  After you’ve added a number that you carried, cross it out so you remember that you used it.   It will still be there for you if you check your work.

The third mistake was less common.   When moving to the 3x29, some students forgot to put their 0 before multiplying (since the 3 really represents 30).  

PPT’s Solution:  Leave some space under the equal line, and put the 0 on the second row (where you would do the 3x29 part) in before you do any multiplying.

They understood why they needed the zero, but they occasionally forgot it.

I also told them they could write “Back on the bottom” on their tests.  I’m looking forward to seeing their grades after their classroom teacher grades it.