4 mistakes learners make with the probability of not happening, each one named and explained.
Everything that can happen adds to 1, so not happening is 1 minus happening.
The answer is the number of students rather than a fraction or decimal.
The count is the natural first step and it is a satisfying whole number.
Question. 30 students, 12 study Spanish
A common answer. 12
The answer. 12/30, which is 2/5
Why. A probability is always between 0 and 1.
If your probability is bigger than 1, you have given a count.
The probability of not raining is given as 0.35, the same as the probability of raining.
The number in the question is the only number available, so it is the one written down.
Question. P(rain) = 0.35
A common answer. P(no rain) = 0.35
The answer. P(no rain) = 0.65
Why. Rain and no rain cannot both be equally likely unless each is 0.5.
The two must add to exactly 1. That is the check, every time.
In a class of 30 with 18 studying French, the probability of Spanish is given as 18/30.
The number named in the question is the one that feels like the subject of it.
Question. 30 students, 18 study French, the rest Spanish
A common answer. 18/30 for Spanish
The answer. 12/30, which is 2/5
Why. The question asked about the twelve who were never counted directly.
Underline the group in the question. The named number is often the wrong one.
1 minus 0.35 is worked out as 0.55 or 0.75, so the two probabilities do not add to 1.
Subtracting a decimal from a whole number needs the 1 written as 1.00 first.
Question. 1 - 0.35
A common answer. 0.55
The answer. 0.65
Why. 0.35 and 0.55 add to 0.9, so one of them must be wrong.
Always add your answer back to the original. Anything other than 1 is a slip.
WAJD spots these patterns in your child's answers and names the one behind their wrong answers, instead of just marking them wrong.