A country's production function works like this each year:

Y = 4K^0.5

It rents capital from the rest of the world at a cost of r per unit of capital per year. Fortunately for us, capital doesn't depreciate in this country, and since firms are profit maximizing, then MPK=r. The marginal benefit of a unit of capital is the MPK and the marginal cost of a unit of capital isr.

If the rental rate of capital is 0.1 (or 10% per year), how much capital will it rent this year, if firms in this country are profit-maximizers?