Hi,
The first thing you have to remember, that the sum of all probabilities are equal to 1.
As you have only 4 items : Pasta + Fish + Pizza +Salad
So pasta + fish + pizza +salad = 1
1 means every client who bought something had something to eat.
So now it simple :
We know the probability of every other items, so we will find the probabilty of salad by substracting off 1 every other probabilities.
<u>Remind yourself to always have data in same shape/ units etc.</u>
Here we have percentage, fraction, numbers.... can be confusing.
So convert all in same shape. I 'm going to convert all in numbers, as it seems to me the simpliest.
I will call probability of salad "X" as it usually like that we call something unknown in math.
So pasta = 15 % = 15/100 = 0.15
fish = 0.2
pizza = 3/10 = 0.3
salad = X
So now we have :
pasta + fish + pizza +salad = 1
replace by what I know :
0.15 +0.2+0.3 + X = 1
0.65 + X = 1
0.65 +X -0.65 = 1 - 0.65
X = 0.35
Salad probability is 0.35 , which means 35 % .
