It is-12 for the determination
The volume of a pyramid is given by:

Where:
• Ab is the area of the base,
,
• h is the height.
In this case, we have:
• a = 3 units,
,
• b = 7 units,
,
• c = 9 units,
,
• Ab = b * c,
,
• h = a = 3 units.
Replacing the data in the formula above, we get:

Answer: 63 units³
- The distance travelled by both Evelyn and Ricky is 45 miles
If Speed = dist/time, then;
<u>dist = speed * time</u>
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We also know that both of them travel the same distance, so we equate the distance travelled to each other
36 * x+45 = 72 * x
36x + 1620 = 72x
72x - 36x = 1620
36x = 1620
x = 1620/36
<u>x = 45 miles</u>
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learn more about distance covered here brainly.com/question/17894416
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He would have 21 before he bought the 10 stamp because 31-10=21