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

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.