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Question
Find the second right most non-zero digit in the number N=24701613.___
Answer:
:
Approach 1: Conventional
24701613=2471613×101613.
The second right most non-zero digit in the number would be the digit at the ten's place of the number 2471613.
2471613100=24716134×25
The remainder when 2471613 is divided by 4 is 3.
So, 2471613=4k+3.
Now we need to find the remainder when 2471613 is divided by 25.
247161325=((2503)1613)25
R=(31613)25=((33)×((310)161))25=(27×(59049161)25)=((2)×(1)16125)
Therefore the remainder is 2. So, 2471613=25n+2.
4k+3 =25n+2.
For k to be an integer, possible values of n are 1, 5, 9, 13...
Corresponding values of k are 6, 31, 56, 81...
Therefore, values of k are of the form: 25m -19.
2471613=4(25m19)+3=100m73.
Therefore, the last two digits is 100-73=27.
So, the second right most non-zero digit in the number N=24701613 is 2.
Approach 2: Using the techique of last two digits
We need to consider only the last two digits.
The number can be written as:
(47)1613=(474)403×47=(472×472)403×47
=(09×09)403×47=(81)403×47=41×47=....27

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