Roland2rule wrote:If s and t are positive integers, st + s + t cannot be

A. 5
B. 6
C. 7
D. 8
E. 9
OA is b

Since the answer choices are small, and since s and t must be POSITIVE INTEGERS, a quick approach is to start testing pairs of values and using the results to eliminate answer choices.
Let's be somewhat systematic.
We'll try, s = 1 and t = 1, then s = 1 and t = 2, then s = 1 and t = 3, then s = 2 and t = 2, etc
Let's go.....

s = 1 and t = 1
st + s + t = (1)(1) + 1 + 1 = 3 (not there)

s = 1 and t = 2
st + s + t = (1)(2) + 1 + 2 = 5
ELIMINATE A

s = 1 and t = 3
st + s + t = (1)(3) + 1 + 3 = 7
ELIMINATE C

s = 1 and t = 4
st + s + t = (1)(4) + 1 + 4 = 9
ELIMINATE E

s = 2 and t =2
st + s + t = (2)(2) + 2 + 2 = 8
ELIMINATE D

By the process of elimination, the correct answer is B

Cheers,
Brent