9
The numbers 503, 509, 521, 523, 541, 547, 557, 563, 569, 571, 577, 587, 593 and 599 are prime.
The number halfway between them is 600.
To find the numbers between 400 and 600 that either begin or end with the digit 5, we can break it down into two parts. First, let's find the numbers that begin with 5: There are 10 numbers between 500 and 599 that start with 5. Next, let's find the numbers that end with 5: There are 10 numbers between 405 and 595 that end with 5. Adding these two sets together gives us a total of 20 numbers between 400 and 600 that either begin or end with the digit 5.
600 * * * * * It is, in fact, 639
400 to 500
503 509 521 523 541 547 557 563 569 571 577 587 593 599
503 and 521.
503 509 521 523 541 547 557 563 569 571 577 587 593 599
500
They are: 529 and 576
The halfway point between 500 and 600 is (500 + 600) ÷ 2 = 550
You can find lists of prime numbers online in many places. Just search Google for "list of prime numbers" or something similar.
9
The numbers 503, 509, 521, 523, 541, 547, 557, 563, 569, 571, 577, 587, 593 and 599 are prime.
100 + 300 + 500 + 500 + 600 = 2000. Then divide by 5 to get 400. The mean of these five numbers is 400.
The number halfway between them is 600.