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For a positive integer n, if 5^n is a factor of 25!, but 5^{n+1} is not a factor of 25!, what is the value of n?
A. 5 B. 6 C. 7 D. 8 E. 9
=>
25! = 1 x 2 x … x 5 x … x 10 x … x 15 x … x 20 x … x 25 = 1 x … x 4 x 6 x … x 9 x 11 x … x 14 x 16 x … x 19 x 21 x … x 24 x 5 x 10 x 15 x 20 x 25 = 1 x … x 4 x 6 x … x 9 x 11 x … x 14 x 16 x … x 19 x 21 x … x 24 x ( 5 x 5 x 2 x 5 x 3 x 5 x 4 x 5^2 ) = 1 x … x 4 x 6 x … x 9 x 11 x … x 14 x 16 x … x 19 x 21 x … x 24 x (2 x 3 x 4 x 5^6 )
There are 5 integers and x≤y≤r≤s≤t. If the median of them is 80 and the average (arithmetic mean) of them is 100, what is the smallest possible value of t?
A. 110 B. 120 C. 130 D. 140 E. 150
=>
In order to have the smallest value of t, we need to maximize x, y and r. Then we have x = 80, y = 80 and r = 80. After that, in order to have the smallest value of t, we need to maximize s and so we have s = 130 and t = 130.
If n is the product of 3 different prime numbers, how many factors does n have except 1 and n?
A. 6 B. 7 C. 8 D. 9 E. 10
=>
n = p*q*r where p, q and r are different prime numbers. Then the number of factors of n is (1+1)(1+1)(1+1) = 8 including 1 and n, since the exponents of p, q and r are 1. Thus the number of factors of n except 1 and n is 8 – 2 = 6.
If 0.010<x-√5<0.011, approximation of 1/√5-1/x is which of the following options?
A. 0.01 B. 0.002 C. 0.02 D. 0.003 E. 0.03
=>
1/√5-1/x = x-√5/x√5
From 0.010<x-√5<0.011, we have 0.010 + √5<x <0.011 + √5. We have 1/0.010+ √5 < 1/x < 1/0.011+ √5 or 1/0.010√5+5 < 1/x√5 < 1/0.011√5+5. 0.010/0.010√5+5 < x-√5/x√5 < 0.011/0.011√5+5. The approximation of x-√5/x√5 is 0.01/5=0.002
In the above table, scores and numbers of people are shown. If the median score is not represented as the score shown on the table, which of the following scores can be the value of x?
A. 2 B. 3 C. 4 D. 5 E. 6
=> The sum of numbers for a+1 and a+2 is 15 = 7 + 8. If 2 + 8 + x = 15, we don’t have the median on the table. Thus, 10 + x = 15 and we have x = 5.