that question needs nothing but an energetic brain..
say you've got a piece of Mg ribbon..about 5 gm.
you've added on it about 15 cm3 of sulfuric acid.
and you know that, that volume of sulfuric acid is the EXACT amount needed for the reaction to occur.
in that question it says that the exact is 20 cm3.
so if you added more than that which is 40 cm3 ..the reaction would occur by the first 20 cm3 and all the magnesium is used up
so the extra 20 cm3 would do nothing but decrease the temperture.
Hope you understood.