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 students procedure and explain why the procedure won't always work.

The mistake the student made in the second procedure is she tried to solve 3.7+2.6 the same way she solved 4.3+8.4, which that way doesn't always work. You have to understand how decimals work when it comes to adding them. The right way to solve 3.7+2.6 and be sure you are accurate is to line the numbers up. Then add the numbers behind the decimal which is 7+6. That equals 13. So you would out your 3 and carry your 1 over to the number in front of the decimal. So 3+2=5, plus the additional one you had to carry. Your final answer would be 6.3