To set up the proportion we first need to find the ratio of the vanilla ice cream sold for the 15 sales mentioned. From the information given we notice that 4 out of 15 were vanilla then:
Now we need to find the second proportion, let x be the number of vanilla ice creams; in this case we have that x out of 105 are vanilla, then:
Equating both ratios we have the proportion:
Solving for x we have:
Therefore we expect that 28 out 105 sales will be vanilla ice cream.