You should try reading notes you took during your class period.
You have two totals: total # of bills and total amount of $. Using x as #of 20s, and y as # of 50s, you can use the system
x+y=18
20x+50y=450
Consider the circle with center X, as shown in the figure.
Draw the diameter of the circle which is parallel to cherd AB, as shown in the figure.
Since the diameter and AB are parallel, then the line segment XC which bisects AB at C, will be perpendicular to AB.
SO triangle XCB is a right triangle. Thus the length of CX, by the Pythagorean theorem is

units.
Answer: 8 units
The median is 42, the lower is 36, and the upper is 86
80% is the score percentage