Which linear relationship can be described as a function with direct variation

the linear relationship that represents a variation in direct proportion is the linear function that varies according to the numbers of its domain, for example the cost of producing x number of items can be represented in direct proportion by f (x) = mx + b, where m is the constant of proportionality, x the number of articles produced and b the fixed cost

rick would give her the best deal as he will charge $8.88 per hour

so, converting firstly the mixed fractions to improper fractions and then "adding".