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Question: A set of steps for the end of a pier are built of stone. A sketch of the cross-section of the steps is shown.

Each step has rise of 0.2m and tread of 0.6m. Form a series to calculate the area of the cross-section.

I tried to answer the question but I got it wrong

I tried to to find the area of A

using Sums of arithmetic - i found the length of the stairs

$\displaystyle S_{12}= \frac{12}{2}\left [ 2(0.6)+(12-1)0.2 \right ] = 19.8$

area of a = $\displaystyle \frac{1}{2}(19.8)(12) = 118.8$

area b = 8 x 12 = 96

therefore total area = 214.8 square metres

the answer in the book is 118.8 square metres

Can someone please help