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:
Step-by-step explanation:
That's just about impossible to determine...
Answer:
5/9
Step-by-step explanation:
<h2>
Answer: a = ¹/₂ (4 + b)</h2>
<h3>
Step-by-step explanation:</h3>
To solve for 'a' we have to make it the subject of the equation. Since there are two unknowns ('a' & 'b'), we won't get a numerical value of 'a', but an expression in terms of the second unknown 'b'.
Since 2a - b = 4 <em> [add 'b' to both sides]</em>
then 2a = 4 + b <em>[divide both sides by 2 = multiplying by ¹/₂]</em>
a = ¹/₂ (4 + b)