Question 94158
1) The sum of three consecutive numbers is odd when the first of these three numbers is even. (8+9+10 = 27)
The sum of three consecutive numbers is even when the first of these three numbers is odd. (15+16+17 = 48)

2) Let x = the tens digit and y = the units digit, then:
x+y = 5 
x-y = 3 or x = y+3 Substitute into the above equation.
y+3+y = 5
2y+3 = 5
2y = 2
y = 1 This is the units digit.
x = y+3
x = 1+3 = 4 This is the tens digit.
The number is 41, a prime number.