Question

A student thinks the sum of 4.3 and 8.4 is 12.7 because 4+8 is 12 and 3+4 is 7. The student then adds 3.7 and 2.6, then gets 5.13 because 3+2 is 5 and 6+7 is 13. Identify the mistake in the student's procedure and explain why the procedure won't always work.

Answer 1

The mistake is that when adding decimals, if the number is greater than 10 then you add one to the ones place and subtract ten from the decimal. So the answer should have been 6.3