Question

If 200 cards are arranged in the given manner,

20 cards are in the bottom row,19 in the next row ,18 in the row next to it and so

on. How many rows will be there?

a) 15 b) 16 c) 20 d) None of these​

Answer 1

Hey.....

The correct answer is b) 16
Here is your answer

It is an AP such that a=20 & d=−1
S
n
​
=200
⇒200=
2
n
​
[2(20)+(n−1)(−1)]
⇒400=n[40+1−n]
⇒n
2
−41n+400=0
⇒n
2
−25n−16n+400=0
⇒(n−25)(n−16)=0
So n=16 or n=25
T
16
​
=a+15d=20−15=5
T
25
​
=a+24d=20−24=−4
Since T
25
​
is not possible.
∴ n=16 & 5 logs are placed in the top row.

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