The expression would be $0.20 x f. Once you figure out what the 'f' variable stands for, you have to multiply it by $0.20, and you will have your answer.
Cell membrane is a semipermable, which means that certain molecules will be able to pass through the membrane. It is contains phospholipids. But for smaller molecules such as CO2 and Water, it can pass through the membrane through simple diffusion. Cell membrane controls the movement of the materials into and out of the cell.
Mountain Bike is the correct answer
