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16 Aug 2019, 00:56
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B
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Difficulty:
75%
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Question Stats:
57% (02:34) correct
43%
(02:40)
wrong
based on 133
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A stairway is formed by stacking even blocks next to and on top of each other. If the stairway depicted above is 5 blocks in length and 4 blocks in height, with a thin step at the beginning that does not require a block, and the stairway is only 1 block in width, how many blocks will be required to build such a stairway 20 blocks in length?
A. 80
B. 190
C. 200
D. 240
E. 400
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16 Aug 2019, 03:31
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Number of blocks required for 5 blocks length staircase= 1+2+3+4= 4*5/2=10
Number of blocks required for 20 blocks length staircase= 1+2+......+18+19= 19*20/2= 190
Number of blocks required for 20 blocks length staircase= 1+2+......+18+19= 19*20/2= 190
Bunuel
04 Dec 2019, 06:25
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Bunuel
Stairway is made of 5 in length, including 1 thin step, so 4 blocks in length by 4 blocks in height.
If you count the example: 4+3+2+1=10 this is an AP of first natural numbers;
Stairway with 20 in length means 19 blocks in length, or the sum of the first 19 natural numbers:
\(n(n+1)/2=19(20)/2=190\)
Ans (B)
