Question

I was having 100 rupees
I spend 40=I was left with 60
I spend 30=I was left with 30
I spend 18=I was left with 12
I spend 12=I was left with 00
So 40=60
30=30
18=12
12=00
__________
100=102
How is that possible

Answer 1

Consider another case

I spend 10 and left with 90.
I spend another 10 and left with 80.
I spend the rest and left with 0.

Now if I calculate the way you calculate it will be

100=170.

Now this clearly doesnt make any sense. So this clearly means that adding the left will not always give the one you started with. So your case is absurd when you calculate the money which is left with you.