1. Unit 7.1
Transparency 2 with Notes for Teachers
Risky behaviour and probabilities
Say: If something has a 100% probability of happening, it is certain that it
will happen. For example, there is a 100% probability that the sun will rise
tomorrow. But, since we can never be absolutely sure about the future, we
don't normally talk about a 100% probability. Who knows, during the night
a large meteor can hit the earth and knock it off its orbit so that tomorrow
the sun won't rise. It is highly unlikely that a meteor will hit the earth, but we
can't entirely rule out the possibility that it will happen. We can only be
almost sure.
• What is the probability of being struck by lighting when you are inside
your home? 99% certain? 50% likely? Or 0%, i.e. certainty that it won't
happen
Answer: we are almost certain that it won't happen, but since we
are never absolutely sure about the future, we can never have a
probability of 0%. Again, we can only say that we are almost certain
that it won't happen.
• What is the probability of being struck by lightning when outside in the
open veld during a thunder storm, in any particular year?
Answer: Since South Africa is a lightning-prone country, with one of
the highest lightning ground flash densities in the world, lightning related
deaths in this country are about four times higher than the global
average. But still, the probability is very small, with about 32 deaths per
year out of a population of 50 million, or a 0.00000064% probability
of being struck dead by lightning in any one year!
Grade 7 | Unit 7.1 | Page 16

2. • What is the probability of a young person developing an alcohol addiction
after drinking a strong brandy?
Answer: Studies have shown that youth are particularly prone to
addictive forms of behaviour. Exploring and experimenting with new
options is part of the development of an adolescent. It can be a useful
learning experience for a young person to get drunk, feel very sick, and
realize that doing so again is not something he or she will choose to do in
future. It is unlikely that a young person, however, will develop a long-term
problem after having had just one drink. But, of course, one drink can lead
to another and if the person drinks regularly and frequently, there is a
much higher probability of a long-term problem.
• What is the probability of an older person in his 60s developing an
alcohol addiction after drinking a strong brandy?
Answer: Studies have shown that the younger the age at which a person
starts drinking, the higher the probability of that person developing a
problem with alcohol. Most adult alcoholics report that they already had
a problem with alcohol at age 17-18. So, the older a person is when they
first start drinking, the lower the probability of that person developing a
problem with alcohol later on.
• What is the probability of a young person developing an alcohol addiction
after getting drunk most weekends?
Answer: as we have discussed in the previous two answers, it is highly
probable that such a young person who drinks regularly and frequently will
have social, emotional and physical problems related to alcohol later on.
There are different probabilities depending on:
• your choice
• your personality
• the circumstances and
• sometimes sheer luck!
Grade 7 | Unit 7.1 | Page 17