Probability that two distinct randomly selected divisors of $70$ differ by an odd number? Each of eight cards has one factor of 70 on it. The eight numbers are all different.
Two cards are selected at random without replacement.
Calculate the probability that the difference of the two numbers on the selected cards is odd??
 A: Hint: Of those 8 cards, there are 4 cards with odd numbers and 4 cards with even numbers. Let's think that the cards with odd numbers are black cards and cards with even numbers are white cards. So we need the probability that we draw two cards without replacement and what's the probability that one of them is black and one of them is white.
Atually the result should be 4/7 if I'm not wrong. because we can take any of the card in first move. We need the probability that in 2nd move the chosen card is not of the same color of the first picked card :) 
A: Consider that whatever first card is selected, of the 7 remaining cards, only 4 would give an odd difference.  Thus the answer is $\frac{4}{7}$.
A: I assume, you mean positive factors of 70. 
70 has only 8 different factors. When you write then down, you will notice, that 4 of them are odd and 4 of them are even. 
Also notice, that p(difference between two card is odd) = p(1. card is odd and 2. card is even) + p(1. card is even and 2. card is odd)
Hint: p(1. card is odd and 2. card is even)  = 4/8 * 4/7
