The equation is 3x = 15. Ishaan writes x = 15 + 3 = 18

Why

Ishaan seems to be following a rule like 'move the number to the other side and change what's happening to it' — a rule that IS real for equations, just applied with the wrong operation here. The '3' in 3x is not being ADDED to x, it's being MULTIPLIED with x — 3x means '3 times x'. To undo multiplication and get x by itself, you do the OPPOSITE of multiplying, which is dividing, not subtracting or adding: divide both sides of 3x = 15 by 3, giving x = 15 ÷ 3 = 5. Adding 3 to 15 to get 18 undoes nothing here, because nothing was ever subtracted from x in the first place. Test the two answers directly: does 3 × 18 equal 15? No, it's 54. Does 3 × 5 equal 15? Yes, exactly. That check alone shows which answer actually satisfies the original equation. His friend is right, x = 5: the operation used to solve an equation must be the OPPOSITE of what's actually being done to x — since 3x means 3 multiplied by x, the equation is undone by dividing by 3, not by adding it.