kevincan
A positive divisor of 4,200 is selected at random. What is the probability that the divisor leaves a remainder of 7 when divided by 14?
A. 1/24
B. 1/12
C. 1/10
D. 1/8
E. 1/6
A positive divisor of 4,200 is selected at random, which means the divisor should be a factor of 4200.
The factors of 4200 are = 6*7*100 = 2^3 *3 * 5^2 * 7
Total number of factors = (3+1)*(1+1)*(2+1)*(1+1) = 4*2*3*2 = 48 factors.
we are asked to find out : The divisor leaves a remainder of 7 when divided by 14.
so, the divisor should be of the form
14K + 7.
At k=0, we get 7,
k=1 , we get 21.
k= 2, we get 35. For subsequent values of K, we won’t find any divisor as factor of 4200.
Is this approach right? No. Does it take much time ? Yes.
Taking 7 as common, we get
7*(2k+1).
So, the divisor should be a multiple of 7 as well as ODD multiple ( 2k+1).
Factors of 4200 = 2^3 *3 * 5^2 * 7
Remove all even numbers = 3*5^2 * 7 = 2*3*2 = 12 odd factors.
Out of the 12 factors, factors which are multiple of 7 are 7*[3 * 5^2].
(2*3)= 6 factors, which are multiples of 7.
Probability = 6/48 =
1/8
Option D