Let $f=f(K,L)$ - production function. Let $f(1,2) \le4 $ and a production function is defined to have increasing returns to scale. What maximum product can the firm produce using K=3 and L=6?
So we know that $f(3,6) = f(3*1, 3*2) > 3*f(1,2)$ and $f(1,2)\le4$
But what should I do next?