Analysis: Profit = selling price-buying price, so asking selling price is the key. Because the price of each piece is 80 yuan, we need to find out the number of T-shirts sold first. Suppose you buy X T-shirts for the first time and 3x T-shirts for the second time. According to the fact that the second time is more expensive than the first time 12 yuan, make another equation.

Solution: Suppose you buy X "T-shirts" for the first time, then

186000/3x-50000/x = 12 .

The solution is x = 1000.

X = 1000 is the root of the original equation.

(1 0,000× 4-400 )× 80+400× 80× 65%-(186,000+50,000) = 72,800 yuan.

A: The mall made a profit of 72,800 yuan on this business.