[問題]天山 04-32

[Janet]

Q32:
If n is an integer between 10 and 99, is n < 80 ?
(1) The sum of the two digits of n is a prime number.
(2) Each of the two digits of n is a prime number.

ans:B

請問本題的解題思路?

[yuying]

(1) 20, 23,...98 等都符合條件, n可能大於80, 或小於80
(2) 如果n兩位數字都需為質數, 則n必須為2x, 3x, 5x, 7x, 因此都小於80

FYR!!

[Janet]

謝謝^^..非常清楚...