Chapter 4.3, Problem 95E

Sociology. Two sociologists have grant money to study school busing in a particular city. They wish to conduct an opinion survey using $600$ telephone contacts and $400$ house contacts. Survey company $A$ has personnel to do $30$ telephone and $10$ house contacts per hour: survey company $B$ can handle $20$ telephone and $20$ house contacts per hour. How many hours should be scheduled for each firm to produce exactly the number of contacts needed?