Suppose the quantity of total capital advanced is 7753. It is divided into 1) the constant capital (c) which is the value of the means of labor (tools, equipment and raw materials) and 2) the variable capital (v) which is the value of living labor. It produces a surplus value (s) of 100 by employing 20 laborers working a 10-hour day for a total weekly wage of 1550.6 .

a) Based on the above information, calculate the rate of profit (r). Show your calculations clearly.

b) Now the working day is lengthened to 15 hours without raising the wages. The surplus value therefore increased by 1.5 times. What will be the new rate of profit?

c) Critically discuss Marx's concept of rate of profit. What is right or wrong with this concept?