Basic Probability
1. The odds of an event happening successfully are expressed as a:b where a:b is the ratio of the
probability of a successful event to the probability of an unsuccessful event. The odds of rolling a 5 on
an unfair die are 1:3. What is the probability of rolling a 5 with this die? Express your answer as a
percent.
Comment [RC1]: 25%
2. Russel rolls a pair of dice with his eyes closed. His sister tells him that the sum of the dice is greater
than or equal to nine. Knowing this information, Russel calculates the probability that he rolled doubles.
What is that probability expressed as a common fraction?
Comment [RC2]: 1/5
3. A bag contains eight white marbles, six red marbles and four blue marbles. How many blue marbles
must be added to the bag so that the probability of choosing a blue marble is ?
4. Mary's class took two test on Friday. Eighty percent of the class passed the math test, 90% of the
class passed the English test and 72% of the class passed both tests. What is the probability that a
randomly selected student in Mary's class failed both tests?
5. The probability that two randomly selected integers from a set have a product that is odd is 1/5. If
the set includes 3 odd integers, how many integers are in the set altogether?
Comment [RC3]: 10 blue marbles
Comment [RC4]: 80-72=8% passed the math
but not English. 90-72=18% passed the English but
not math. 100-(18+8+72)=2% did not pass either
test.
Comment [RC5]: 6 integers. Since only odd x
odd= odd, there are three combinations out of 15
combinations which can be made with 6 integers.