**Solution for yesterday’s problem**

Let the selling price of 12 marbles is 100 cents

at 20% loss, the cost price = (100/80)*100 = 125 cents

at 20% gain, the selling price of 12 marbles = (120/100)*125 = 150 cents

So, for 150 cents he sells 12 marbles

Therefore, for 100 cents he should sell (12/150)*100 = 8 marbles.

**Fastrac method**

Let the selling price of 12 marbles at 20% loss is 100 cents

Therefore, he bought 10 marbles for 100 cents: (100/120)*12 = 10 marbles

In order to gain 20% he should sell: (80/100)^10 = 8 marbles

(If he sells 20% more marbles for 100 cents he will lose 20% and if he sells 20% less marbles for 100 cents he will gain 20%.)

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