What number must be added to 15 -6 and 12 to have a mean of five

Which shows the correct solution of the equation 1/2a+2/3b=50, when b=30?

The cost of using a service is $0.25 per min. what equation correctly represents the total cost c, in dollars, for d days of use?

He ! a group of farmers planned to clean 80 acres of wheat per day to finish the work on schedule. right before the work started, they received a new piece of machinery that them clean 10 more acres per day than originally planned. as a result of increased productivity in the last day before the deadline they only had to clean 30 acres. how many acres of wheat did the group of farmers have to clean?