Consider the following information about a firm:

Marginal product of capital in the next year: MPK = 206 - 4K units of output per additional unit of capital per year

Real interest rate: r = 0.05 per year

Depreciation rate of capital: d = 0.10 per year

Price of capital: p_K = 200 units of output per unit of capital

Capital stock at beginning of year: K0 = 40 units of capital

Assume now there is a 40% tax on firms' output and depreciation and interest payments are not tax deductible. What is the desired level of the capital stock at the beginning of the next year?