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Using the ladder shown in the figure, if the bottom rung is 45 cm long and each successive rung is 2 cm shorter than the previous one, what is the length of the 8th rung from the bottom? 
A33 cm
B31 cm
C29 cm
D35 cm
Answer & Solution
Correct answer: B. 31 cm
The rung lengths form an AP: $45, 43, 41, \ldots$ with first term $a=45$ and common difference $d=-2$. The 8th term is $a_8=a+(8-1)d=45+7(-2)=45-14=31$. So the 8th rung is 31 cm long.
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