Let
be the random variable for the number of marks a given student receives on the exam.
10% of students obtain more than 75 marks, so

where
follows a standard normal distribution. The critical value for an upper-tail probability of 10% is

where
denotes the CDF of
, and
denotes the inverse CDF. We have

Similarly, because 20% of students obtain less than 40 marks, we have

so that

Then
are such that


and we find

Answer:
7.18 light-years
Step-by-step explanation:
6.8 x 10^13 = 68,000,000,000,000
9.46 x10 ^12 = 9,460,000,000,000
6.8 x 10^13 * 1 light year / 9.46 x 10^12 = 7.188 light years.
Have a Merry X-MAS Plz MArk Branilest