Making a model is simple. You could use a number line or draw one out. I could help you, but I don't have a problem to work with...
Answer:
2.5
Step-by-step explanation:
The formula to find the midpoint of a line segment: 
<u><em>Where:</em></u>

1) Substitute the values into the midpoint formula.

2) Solve it.
= 
= (2.5, 8.5)
D.d=24%
d.r=29%
d.t=53%
r.d=24%
r.r=24%
r.t=47%
t.d=47%
t.r=53%
t.t=100%
Just take the numbers and divide them over the total amount which in this caase is 51.
320 times 0.14 = 44.8
So 45 people are over the age of 65
Volume is calculated by multiplying (3/4)pi by the radius cubed