mean time taken to solve prob (need help w/ starting)

[zorro]

Question :
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 )

[bobbym]

Hi zorro;

i) 120 to 154 represents -2.06666 sd below the mean and .26666 sd above the mean. The area under the curve that represents the standard normal curve is:



Meaning that 58.57% of the students fall into this range.



So 292.86 students take between 120 and 155 seconds.

ii) 185 is 2.2666 sd above the mean. This represents .0117 or 1.17 % of the students.