Constructed response 1: a student thinks that the sum of 4.3 and 8.4 is 12.7 because 4 + 8 = 12 and 3 + 4 = 7. the student then adds 3.6 and 2.7 and gets 5.13 because 3 + 2 = 5 and 6 + 7 = 13. identify the mistake in the student’s procedure and explain why this procedure won’t always work.

The mistake is that given that 6 and  7 are in the place of tenths, 6 tenths + 7 tenths means 13 tenths and that is 1 unit plus 3 tenths.

So 3.6 + 2.7 = 5 units + 1 unit + 3 tenths = 6 units + 3 tenths = 6.3

If you write 5.13 that is 5 units and 13 hundreeths which is wrong.

The procedure used by the student wont work when the addtion of the two digits is 10 or greater than 10. In those cases, you must pass the left digit to the next place and sum it to the other digits that are in that place since the beginning.