50 coins are there. 25 coins are normal coins weighing 10 gm each. Other 25 coins are faulty coins. some of faulty coins weigh 9 gms each and some faulty coins weigh 11 gm each. You have a balance scale with two pans. In each weighing the scale shows the difference of the weights placed in each pan. You will randomly choose a coin. What is the minimum number of weighings necessary to guarantee to determine whether it is normal or not?