Stacy multiplied 5x9 and said the answer was 44. What two patterns could you use to show that the answer is not correct?
I suggest the following:
1. When 5 is multiplied by an integer, the result always ends in a 0 or a 5. (44 does not end in a 0 nor a 5).
2. When 9 is multiplied by an integer, the resulting digits always add up to a multiple of 9. (4+4=8 is not divisible by 9).