We have 3 elements here CP-cost price, SP-selling price, and profit percentage Profit%.
So the equations relating them are:
Profit=SP-CP (for loss its CP-SP)
Since we don’t know the cost of the article it is ok to assume a value initially.
Let the CP of each article =1 unit.
CP of 20 articles =20 units
SP of x articles=CP of 20 articles= 20 units
SP of 1 article = 20/x
using formula Profit%=((SP-CP)/CP)*100 for 1 article as profit% is a constant for each article.
=> 25x/100 = 20-x
=> x/4 = 20-x
solving we get x=16 units.