A shopkeeper calculate percentage profit on the buying price and another on the selling price. What will be their difference in profits if both claim a profit of 20% on goods sold for Rs. 3000?

For 20% profit on selling price means $$\frac{1}{5}$$ of 3000 i.e. Rs. 600
Now, let the CP = Rs. 100, Then,
SP with 20% profit = Rs. 120
For 20% profit on selling price means cost is 100 + profit is 20 = selling price is 120.
Means selling price is 120% of cost price.
Now selling price is 120% ie 3000 then find 100% amount which will be cost.
Cost = $$\frac{{3000}}{{120}}\% $$
= $$\frac{{3000}}{{\frac{6}{5}}}$$
Because, 120% = $$\frac{6}{5}$$
= 3000 x $$\frac{5}{6}$$
= 2500
Cost is 2500
Thus profit is 20% i.e. $$\frac{1}{5}$$ x 2500 = 500
Thus,
Difference is 600 - 500 = Rs.100