The conversion factor of ounces to pound is 16 ounces per 1 pound. The ratio and proportion concept is used here. The ratio of the price of broccoli per pound should be the same with that of the price of 64 ounces.
($1.50 / pound) x (1 pound/ 16 ounces) = (P / 64 ounces)
The value of P in the equation is 6. Thus, the price of 64 ounces of broccoli should be $6.