Question

if 4 cooks take 4 hours to bake 4 cake, then how long will it take ( assuming no change of rate) 8 cooks to bake 8 cake ?

Answer 1

4 cooks take 4 hours to bake 4 cakes;
4*2 = 8 cooks will take 4/2 = 2 hours to bake 4 cakes;
8 cooks will take 2*2 = 4 hours to bake 4*2 = 8 cakes.

Or simply twice as many cooks will make twice as many cakes in the same amount of time.

Answer 2

it will take 4 hours to bake 8 cakes by 8 cooks