How many 4-digit numbers (ABCD) can be formed such that |A – D| = 2? Choose the correct answer.
A) 2000	B) 1900	C) 1800	D) 1600	E) 1,500

Rationale: digit a can be filled in 9 ways ( excluding 0 ) digit b can be filled in 10 ways digit c can be filled in 10 ways digit d can be filled in 2 ways ( because of modulus a - d ) the answer will be 9 * 10 * 10 * 2 = 1800 ans c
Answer: C