Answer:
No, she is not correct as her speed is 0.25 miles per minute.
Step-by-step explanation:
Given:
Ratio of distance to time taken is 15 : 60.
Distance is in miles and length of time is in minutes.
We know that, average speed is given as the ratio of the distance traveled and the length of the time taken.
So, the ratio above is nothing but the average speed of Reese for cycling.
Thus, average speed of Reese is given as:

Therefore, the average speed of Reese is 0.25 miles per minute which is not equal to the one mentioned by Reese as 4 miles per minute.
So, Reese conclusion of her average speed is incorrect. It's not 4 but the reciprocal of 4 which is 0.25 miles per minute.
They will require between 18.42 and 31.58 minutes.
We want the middle 90%. The two probability values associated with this would be 0.95 and 0.05; this would leave 5% below and 5% above, giving us the middle 90%.
Using a z-table (http://www.z-table.com) we see that the z-score associated with an area to the left of 0.05 is between -1.64 and -1.65; since it is equally distant from both we will use -1.645.
The z-score associated with an area of the left of 0.95 is between 1.64 and 1.65; since it is equally distant from both we will use 1.645.
The formula for a z-score is
z = (X-μ)/σ
-1.645 = (X-25)/4
Multiplying by 4 on both sides,
-1.645(4) = X-25
-6.58 = X-25
Adding 25 to both sides,
-6.58+25 = X
18.42 = X
For the upper bound,
1.645 = (X-25)/4
Multiplying both sides by 4,
1.645(4) = X-25
6.58 = X-25
Adding 25 to both sides,
6.58+25 = X
31.58 = X
The times are between 18.42 and 31.58 minutes.
We know that 80% of the stamps are US and Mexican.
The US stamps are 3 times the Mexican stamps. That means that of the 80%, Mexicans comprised of the 20% and US comprised of the 60% to satisfy the relationship of their ratios.
Therefore, 60% of Michelle's collection is made up of US stamps.