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.
110...... 500 to 599=100 405.415 till 495
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.