Why are we comparing the ratios of MPL to price of labor and MPK to price of capital not ratios of MPL and MPK to price of labor and price of capital

Question:

A firm produces chairs using labor and capital. The price of labor is \$50 dollar per unit, and the price of capital is$100 per unit. At current output, the marginal product of labor is 10 chairs, and the marginal product of capital is 15 chairs. To reduce the total cost of producing the current quantity of chairs, how should the firm change its spending on labor and capital?