Which of the following is the next number in the series 3,6,4,9,5,12,6,_

Two blends of tea are mixed to make a blend that will be sold at the fair. black tea sells for $2.20 per pound and orange pekoe tea sells for $3.00 per pound. how much of each should be used to obtain a 4lb blend selling for $2.50 per pound?

How do you solve a question like this?

Adj has a total of 1075 dance and rock songs on her system. the dance selection is 4 times the size of the rock selection. write a system of equations to represent the situation.