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.