Amateur wrote:
number of factors and prime factors is fine.... but out of those number of factors... how did you conclude on as only 3 being odd?
(Will try to explain this using an easier but slightly slower approach since many have difficulties grasping perms & combs)
Using the number 72 from the original question;
[1] Find number of factors:
1. 72 = 2^3 x 3^2;
2. therefore number of factors = (3+1) x (2+1) = 12
[2] Find number of ODD factors:
*Number property, N1: We know that all primes, except 2, are odd.
*Number property, N2: We know that ODD x ODD = ODD.
*Number property, N3: Multiplying any number by 2 (an Even Number) will yield an EVEN number.
1. Recalling from [1], we have identified 2 and 3 as the prime factors of 72.
2. We ignore the "2" remembering N3.
3. We can construct factors that consist of prime factor 3 only:3, 3 x 3 (since there are only two "3"s we stop here).
4. Let's not forget that "1" is also a non-even factor.
5. Total of ODD factors is 3.
[3] Find number of EVEN factors: Total of factors - total of odd factors = total of even factors = 12 - 3 = 9.
One could directly use combinations of 2, 2, 2, 3, 3 to list all EVEN factors but I've found it faster to find ODD factors first.
For example, in my explanation for counting factors for a number with three distinct primes:
1. 360 = 5 x 8 x 9 = 5^1 x 2^3 x 3^2
2. Number of primes = (1+1) x (3+1) x (2+1) = 24
3. Number of odd factors will be multiples of only up to 1 "5" and 2 "3"s.
4. List odd factors:
3
5
9 = 3 x 3
15 = 3 x 5
45 = 3 x 3 x 5
5. Do not forget that "1" is also a factor, therefore there are 6 ODD factors in 360.
6. Total of EVEN factors in 360 is 24 - 6 = 18.
*Quick Check with pairs indeed reveals 6 ODD factors:
1, 360 --> 1 is ODD
2, 180
3, 120 --> 3 is ODD
4, 90
5, 72 --> 5 is ODD
6, 60
8, 45 --> 45 is ODD
9, 40 --> 9 is ODD
10, 36
12, 30
15, 24 --> 15 is ODD
18, 20
Hope this clarifies things.
* Do note that I would expect 750+ questions to involve combinatorics that involve the use of perms & combs to solve within the time limit.