Which number is prime?
a.
57
b.
63
c.
51
d.
59

It's D. 59, because all of the other numbers are divisible by 3, and you can tell by adding up the digits. (5 + 7 = 12, 3 × 4 = 12) (6 + 3 = 9, 3 × 3 = 9) (5 + 1 = 6, 3 × 2 = 6)

38+105= 143, 39+104=143, and you will find then adding the numbers on opposite sides, moving inwards yields the answer 138 every time. because of this, you’re going to multiply 138 times the number of pairs, which is 34. therefore, 34*138= 4692(i’m sorry if it’s the wrong numbers, but it’s the right concept)