10, 1, 8, 5, 2, 20, 40, 16.
No - the closest triangular numbers either side of 80 are 78 and 91.
Well, let's see. 80 = 20 x 4. 80 = 10 x 8. 80 = 4 x 20. etc. etc.
80 is not a prime number. It is a composite number. Definition: A prime number has exactly two factors: 1 (also known as unity) and itself. A composite number has more than two factors; in other...
4 x 20 = 80