It is a nice little question to pick up two valuable marks, especially if near the level 3/4 or 4/5 boundaries, but one again, more than one calculation has to be made to reach the correct answer.
The first thing to do is work out that two of the lighter grey rectangles will total 40 cm. Then subtract 40 cm from 45 cm, leaving 5 cm which is the answer to 13a.
The second part can be worked out in several ways, including:
1. Subtract 5 cm from 20 cm, leaving 15 cm. (Here is the reason for the optional mark if the answer is incorrect as it shows an understanding of the problem.)
2. Work out that 3 of the light grey rectangles will total 60 cm. Subtract 45 cm from 60 cm which gives 15 cm.
One mark is awarded for the correct answer to 13a.
One mark is awarded to the correct answer to 13b.
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