Solving 18 − 6 − 2, Faisal first works out 6−2=4, then does 18−4=14

Why

Faisal is grouping the last two numbers together the way you might group numbers in an addition sum, where it genuinely doesn't matter which pair you add first — 2+3+4 gives the same answer whichever two you combine first. Subtraction doesn't share that freedom: 18 − 6 − 2 is a chain that must be worked strictly left to right, taking away 6 first, THEN taking away 2 from what's left, not grouping the last two numbers as if they form their own separate step. Done correctly: 18 − 6 = 12, then 12 − 2 = 10. Faisal's method computed 6 − 2 = 4 first and then did 18 − 4 = 14, silently changing what's being subtracted from what. Test: imagine ₹18 in your pocket, spending ₹6 on one thing and then ₹2 on another — you'd naturally track it step by step, ₹18 down to ₹12 down to ₹10, not compute the difference between the two purchases first. His friend is right, the answer is 10: a chain of subtractions (or of subtraction and addition) is worked left to right, because unlike addition, regrouping which pair you calculate first changes the answer.