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I must have made a mistake, because the only way for it to work out is to have a negative length for RS ;)

I'll check again later when i have pen and paper in front of me.

Oh, my bad. I forgot the overlap between the two top trapezia in Figure 2. My answer in (a) was 42 (@Aardvark~~), instead of 21.

Answers:
(a) 21 cm
(b) 13 cm

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Odd, I must be making the exact same mistake.. I've done it several times now and I also keep coming up with 42..... Are you sure it was a mistake?

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Yes, 21 cm is the correct length.

3 times the perimeter of a single trapezoid is 288 cm.

288-204 = 84cm. 84 cm corresponds to 4 times the length of PQ. Twice for the overlap between the top 2 trapezoids in Fig. 2 and twice for the overlap between the top left and the trapezoid just below in the same figure.

84/4=21 cm.

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Oh, I see where you made the mistake. You did actual math instead of just guessing 42....

Don't worry, it happens to the best of us.

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You got me ;)

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No worries, I will tag you when I have a question that requires 42 as the answer

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Splendid!

Sensei approves

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