There are actually two ways given to add or subtract significant figures in different books .In books like concepts of Physics by HC Verma it is written that round of the numbers given first and then add or subtract them but in others book it is written to add the numbers first and then round of the answer. Usually the answer is same but sometimes the answer is different for example while adding 24.36 + 0.0623 + 256.2 answer will come different both the times when we perform by both ways.