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Bunuel
Since we have an AP series (14,16..) after the first operation. On using the formula for nth term. I am getting the nth term as 208. (where the first term is 14, d=2 and n=98.). What is it that I am missing out?
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Bunuel
Since we have an AP series (14,16..) after the first operation. On using the formula for nth term. I am getting the nth term as 208. (where the first term is 14, d=2 and n=98.). What is it that I am missing out?

Including 14, n comes out to be 99 not 98. You can try smaller number of & to test your logic.
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Hi Bunuel,

i don't understand the 2nd step.

one & is already consumed for m=15 and now remaining & are 98 time then in order to calculate 98th term of & then as per formula a98= a+ (n-1)*d

here a= 14, d=2 and n-1=98-1=97

then 98th value of & should be =14+ 2*97=208

Please correct me if I am wrong
Bunuel
Official Solution:


For all positive integers \(m\), \(\&m = m - 1\) when \(m\) is odd and \(\&m = m + 2\) when \(m\) is even. What is the value of \(\&(...\&(\&(\&(15)))...)\), where \(\&\) is used 99 times?


A. 120
B. 180
C. 210
D. 225
E. 250


Since 15 is odd, then \(\&15 = 15-1=14\).

Now, we need to calculate \(\&(...\&(\&(\&(14)))...)\), where \(\&\) is used 98 times.

Since 14 is even, each \(\&\) just adds 2 to the previous result. Therefore, \(\&(...\&(\&(\&(14)))...)\), where \(\&\) is used 98 times, equals to \(14 + 2*98 = 210\).


Answer: C
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aronbhati
Hi Bunuel,

i don't understand the 2nd step.

one & is already consumed for m=15 and now remaining & are 98 time then in order to calculate 98th term of & then as per formula a98= a+ (n-1)*d

here a= 14, d=2 and n-1=98-1=97

then 98th value of & should be =14+ 2*97=208

Please correct me if I am wrong
Bunuel
Official Solution:


For all positive integers \(m\), \(\&m = m - 1\) when \(m\) is odd and \(\&m = m + 2\) when \(m\) is even. What is the value of \(\&(...\&(\&(\&(15)))...)\), where \(\&\) is used 99 times?


A. 120
B. 180
C. 210
D. 225
E. 250


Since 15 is odd, then \(\&15 = 15-1=14\).

Now, we need to calculate \(\&(...\&(\&(\&(14)))...)\), where \(\&\) is used 98 times.

Since 14 is even, each \(\&\) just adds 2 to the previous result. Therefore, \(\&(...\&(\&(\&(14)))...)\), where \(\&\) is used 98 times, equals to \(14 + 2*98 = 210\).


Answer: C

You are confusing "using & 98 times" with "finding the 98th term."

After applying & to 15 once, we get 14, that is already the first term. Applying & again moves to the second term, and so on. So if you want the result after using & 98 more times, you would actually reach the 99th term, starting from 14. That is why we directly calculate: 14 + 2*98 = 210.

Your approach is wrong because you treat 14 as the 0th term, not the 1st.
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I like the solution - it’s helpful.
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This is a great question that’s helpful for learning. Its not a easy question.
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