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