Answer:
The cost of mixture is

Explanation:
We are given the following in the question:
Cost of white per gallon = $12 a gallon
Amount of white paint in the mixture = m gallons
Total cost of white paint =

Cost of blue per gallon = $30 a gallon
Amount of blue paint in the mixture = n gallons
Total cost of blue paint =

Total cost of mixture =
Total cost of white paint + Total cost of blue paint

Total amount of mixture =
Amount of white paint in the mixture + Amount of blue paint in the mixture

Cost of mixture per gallon =

is the required cost of mixture.