How to Determine the Probability of Events for the GED Science Test

By Murray Shukyn, Achim K. Krull

Probability is a concept you will definitely want to be familiar with for the GED Science test. Probability is the likelihood of one or more events occurring. The probability of the sun rising tomorrow morning is almost a certainty. The probability of your winning the lottery tomorrow is much less likely (and is zero if you didn’t buy a ticket). Here, you learn how to calculate probability for simple (one-time) events and compound (two or more) events.

Simple events

Simple events are independent. No matter how many coins you toss or how many times you flip a coin, with each toss of the coin, you have a 1 in 2 chance of it landing on heads.

To calculate probability, divide the number of ways the desired outcome can happen by the total number of outcomes possible. For example, a die has 6 sides marked with dots representing the numbers 1 through 6, so the total number of outcomes possible is 6.

The chances of rolling a 5 (desired outcome) are 1 in 6 or 1/6, because it can happen only 1 way — if the die shows a 5. A 52-card deck of playing cards has 4 aces, so there are 4 ways to draw an ace (desired outcome) and 52 possible outcomes when you draw a card from the deck, so the possibility of drawing an ace is 4/52 = 1/13.

To calculate the probability of either of two or more events occurring, add the probabilities of the two events. For example, what are the chances of rolling a die and having it come up a 2 or a 5? Each event has a 1 in 6 chance, so add the probabilities:

Compound events

Compound events are two or more events occurring at the same time or sequentially. For example, what are the odds of a coin landing on tails 6 times in a row? To calculate probability for compound events, multiply the probabilities of each event occurring. For example, each time you toss a coin, you have a 1/2 chance it’ll land on tails, so the chance of tossing 6 tails in a row is:

Calculating probability of compound events becomes more complicated when an event changes the odds for the next event. For example, to determine the probability of drawing 4 hearts from a standard 52-card deck of playing cards, you need to subtract the number of cards drawn from the total. The odds of drawing a heart on the first draw is 13 in 52 (or 1 in 4).

Assuming you drew a heart with your first draw, there are now 12 hearts and a total of only 51 cards in the deck, so the chance of drawing a heart on the second draw is 12 in 51. On the third draw, you have an 11 to 50 chance.