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kevincan
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percyboi66
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kevincan
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From the total number of factors, subtract the number that have at most three positive divisors.
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Use the formula for total number of factors of a number by prime factorizing.

1800 = 2^3*3^2*5^2.
Total factors = 4*3*3 = 36.

Now you need to remove those factors whose number of factors of those factors are 3 or less than 3.
When does that happen?
When you have a^2 form or just a form. Every other face you will have more than 3 factors for that factor.
Also dont forget about 1.

Now from prime factorization of 1800 we can remove the single factors ie- 2,3,5 which is 3.
We should also remove a^2 form which are 2^2,3^2,5^2 which is another 3.
Including 1 it is 3+3+1 = 7 factors removed.

Final answer = 36-7 = 29.
percyboi66
I got the correct answer (D) but had to take a very roundabout way by basically writing down all the factors of 1800, is there a quicker way?
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total factors of (1800=2^3*3^2*5^2)=36
out of these ones which don't have more than 3 factors are,1,2,3,4,5,9 & 25
hence required ones=36-7=29
D
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Sol:

As, 1800=2^3 x 3^2 x5^2
Total factors= 4x3x3 = 36
No. of Prime factor(i.e 2,3,5) = 3 (having 3 positive divisors)
No. of Prime factor(i.e 4,9,25) = 3 (having 3 positive divisors)
Ans = Total factors - prime factors-square of prime factor - 1(1 having only 1 divisor)
= 36 - 3- 3 - 1 = 29 (D)
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