i) When u use four grams it reduces the temp by 4 degrees. More heat is taken from the surroundings in when u add a greater mass of the substance B, reducing the temperature even further....precisely doubling the mass would cause double decrease in temp. Directly proportional
ii) when there was 30cm3 of water, heat could be taken only from that small amount of water, causing a drop of 6 degrees. More the water, the drop in temp would be lesser, as heat for the reaction is obtained from a larger volume for the same amount of solid as earlier. And since double the volume, half the temp. Inversely proportional
I explained iii) above itself
