The mean time taken by 500 students to solve a problem is 151 sec and the standard deviation is 15 sec . Assuming that the times are normally distributed , find how many students take

i) between 120 and 155 sec.

ii) more than 185 sec.

Solution :

n = 500

= 151

s = 15

i) between 120 and 155 sec

_{(is this correct ?)}

(kindly just tell me which formula to use so that i can get started )