Mr. Thomas sold 20 articles for Rs.60 and thus, gained 25%. How many articles should he sell for Rs.90 so that he will incur a loss of 25%?

Step 1: First, we need to find the cost price of one article. We know that Mr. Thomas gained 25% profit by selling 20 articles for Rs.60. So, the cost price of 20 articles is 100% and the selling price is 125% (100% cost price + 25% profit).

Step 2: Therefore, the cost price of 20 articles is (100/125) * 60 = Rs.48.

Step 3: Hence, the cost price of one article is 48/20 = Rs.2.4.

Step 4: Now, we need to find out how many articles Mr. Thomas should sell for Rs.90 so that he incurs a loss of 25%. This means the selling price of the articles is 75% of the cost price (100% - 25%).

Step 5: Therefore, the selling price of one article is 75% of Rs.2.4, which is (75/100) * 2.4 = Rs.1.8.

Step 6: Finally, the number of articles he should sell for Rs.90 is 90/1.8 = 50 articles.