I believe in order to solve this question, use L'Hopital's Rule, as both the top and bottom will equal 0/0. Which is not what we want. Basically take the derivative of the top expression and take the derivative of the bottom expression. So the derivative of top can be written as d(X)/dx - d(5)/dx and divide everything by the derivative bottom expression that you must take as d(X+4)^1/2/dx, as square root of X+4 is the same as (X+4)^1/2. So altogether it would be d(X+4)^1/2/dx - d(3)/dx. So after taking the derivative on the top it becomes 1-0= 1. Then taking the derivative of bottom will be 1/[2(X+4)^1/2] - 0 = the same thing. Plugging in 5 into the new expression with derived top and bottoms would be 1/1/[2(5+4)^1/2. Evaluating will give you the answer as 1/6.
A normal curve has approximately 95% of graph between mean - 2sd and mean + 2sd So 95% of the times will be between 0 and 12 minutes. 6 - 2x3 to 6 + 2x3 2.5% will take over 12 minutes
Strangely 2.5% will also take less than 0 minutes to process which shows the normal curve is not perfect in this example.
