Asked to simplify 4x + 2x, Rohit says the answer must be a single number, so the question is…

Why

Rohit is expecting every maths question to end in a plain number, the way arithmetic does. But 'simplify' asks something different: write the same thing in a shorter form, not find a value. 4x and 2x are like terms, both counting x's, so four x's plus two x's makes six x's: 4x + 2x is 6x. That IS the finished answer; it stays in terms of x because the question never said what x is, and it does not need to. Test: if you are later told x is 5, then 6x is 30, and 4 times 5 plus 2 times 5 is 20 + 10, which is also 30 — 6x already holds the answer for every possible value of x at once. Meera is right, the answer is 6x: an algebraic expression is a complete answer on its own, and it only turns into a single number once you are given a value for the letter.