2520/2250=28/25=1.12, so the profit is 1.12-1=0.12 which is 12%.
Another way to solve this is to work out the actual profit in money: 2520-2250=270 sh.
The percentage profit is 270/2250=3/25=0.12=12%. Note that the profit is measured against the buying price.
To make a profit of 20% we need to multiply 2250 by 1.20=2700 sh.
Or, work out 20% (⅕) of 2250=450 sh and add to 2250=2700 sh.