Miles draws three cards at random from a standard deck of 52 cards, without replacement. (a) Find the probability that all three cards are red.

(b) Find the probability that all three cards have the same rank.

P( red, red, red no replacement ) =  2/17

P( 3 cards with same rank, no replacement) =  1/425

Step-by-step explanation:

There are 52 cards, 26 are red

P( red) =  red/total =26/ 52 = 1/2

Now draw the second card without replacement

There are 51 cards, 25 are red

P( red) =  red/total =25/51

Now draw the third card without replacement

There are 50 cards, 24 are red

P( red) =  red/total =24/50 = 12/25

P( red, red, red no replacement ) = 1/2 * 25/51 * 12/25 = 2/17

P ( all have the same rank)

There are 52 cards,  4 of each rank

We don't care what the first rank is, now the second and third draw have to have the same rank as the first

P( rank) = 1/1

Now draw the second card without replacement

There are 51 cards, 3 are left with the same rank

P( same rank) = cards with same rank/total = 3/51

Now draw the third card without replacement

There are 50 cards, 2 are left with the same rank

P( same rank) = cards with same rank/total = 2/50 = 1/25

P( 3 cards with same rank, no replacement) = 1 * 3/51*1/25 = 1/425

True

False

Step-by-step explanation:

a) The first statement is the basis for Euclid's algorithm to compute the gcd of two nonnegative integers a,b. You can prove this as follows.

Let G=gcd(a,b). Since r=a-bq and G divides a and G divides b, then G divides r. Now, G divides a and G divides r, hence G divides gcd(b,r).

On the other hand, since a=bq+r, and gcd(b,r) divides b and r, then gcd(b,r) divides a. Therefore gcd(b,r) divides a and b, which implies that gcd(b,r) divides G.

x divides y and y divides x implies that |x|=|y|. The GCD's are nonnegative, therefore G=gcd(b,r).

b) It is false. In general, to test for primality of N, you have to check that all primes smaller than N do not divide N. In this case, we have to check for 2,3,5,7,11,13,17,19,23,...

101 is prime, but this may be false in general. For example, consider N=13*11=143. N is not prime, and n is not divisible by 2,3,5, or 7.