Let x be one number and y the other. Make y the subject. Differentiate the sum with respect to x and substitute y^2 with (60-x)^2. Find x when dy/dx=0 at stationary value (minimum). Find y using the eqn at the top. Find minimum sum. Not sure if you need to prove the sum is minimum but if so then find dy/dx when x=30.01 and when x=29.99. Draw the gradient and you shld be able to show its a minimum :) hope I helped!