Mathematics for Dyslexics and Dyscalculics, 4th Edition

13Decimals

Introduction

Decimal numbers are an extension of the place value of our base ten system. They follow the same logic, but often the perceived inconsistencies block understanding. The processes of addition, subtraction, multiplication and division are identical to those covered for whole numbers in Chapters 8–10, but the decimal point confuses learners. This is not helped by books that suggest, for some calculations, ‘Move the decimal point.’

As with all chapters in this book, we will try to encourage an understanding of the work, partly by relating it to and building on previous concepts and knowledge.

Addition and Subtraction

Addition

Common errors tend to be due to misalignment of the decimal point, that is, incorrectly lining up the place values of the numbers involved (Ashlock, 2010). The tendency persists to line up the numbers from the right, irrespective of place value. Reinstating place value column headings can help.

An example of a common error pattern is:

The correct process can be illustrated by using Dienes base ten blocks or similar apparatus. Effective help can also be provided with the use of money. For example, students who begin by adding £1.23 + £5 with coins will rarely try to add the 5 and the 3, as in the written version above. Reinforcement can be by using the suggestion that £5 may be written as £5.00, writing the .00 because it has ...