Wendy is good at English and Chinese, but her math isn't good. She doesn't think it's useful. 

The clothes at Wendy's mom's shop are on sale today, and there are many people. Her mom wants to teach her a lesson, so she tells her to help sell (售卖) the clothes at the shop. A woman in red walks in and wants to buy a sweater. She chooses between the yellow one and brown one for some time, and says, "I'll take this yellow orle. What's the price (价格) of it?" Wendy says, "It is 400 yuan, but it's on sale. You can take it at haff price. That's 200 yuan." "I see. This is my VIP card. I can get a 20％ discount (折扣), right?" "Oh, yes," Wendy says. "Then, 40 yuan is OK." The woman looks at Wendy in surprise, saying. "Are you sure?"

Mrs. Black, Wendy's mom, comes and says to the woman, "I'm sorry. It is160 yuan, please." Then, she looks at her daughter and asks. "Now, do you think math is useful?"