nick1816 wrote:
The registration number of Lisa's car is a 10 digit number. Though she forgot the last 4 digits, but she knew that product of those digits is an odd number which has more than two factors. How many trials would guarantee access to the online details of the vehicle?
1. 608
2. 610
3. 612
4. 624
5. 625
This means all the 4 digits are odd..
So any of 1,3,5,7,9..
If there were no restrictions, we would get possible ways 5*5*5*5=625..
But there is a restriction of more than 2 factors, this means it cannot be a prime number..
Possibilities of product to be a prime.
1) All of them are 1.....1*1*1*1.....ONE way
2) 3 are 1, and the fourth is a prime so 4!/3!=4..3! for three 1s.
Since there are 3 primes, 3,5,7, the ways become 4*3=12..
Total ways not possible 1+12=13..
Answer with restrictions =625-13=612
C
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