Plz help with this question

2-digit numbers less than 91 which are 1 less than a multiple of 10

I’m stuck on this equation, anyone got the answer?

Kayla made observations about the sellin price of a new brand of coffee that sold in the three different sized bags she recorded those observations in the following table 6 is $2.10 8 is $2.80 and 16 is to $5.60 use the relationship to predict the cost of a 20oz bag of coffee.