Olympiad math problem! Urgent need!

Shopping in 450 yuan, 450÷200=2 over 50.

Shopping vouchers can be obtained: 2× 100=200 yuan.

A * * * can be bought: 450+200 = goods from 650 yuan.

Over 200, issued 100.

Then buy 200+ 100=300 yuan goods, only need to spend 200 yuan money.

900÷300=3

It costs at least 200×3=600 yuan to buy goods from 900 yuan.

2.

If the purchase price is 1, the price after 20% increase and 10% discount is:

( 1+25%)×0.9= 1 12.5%

At this time, the price of each pair of shoes exceeded the purchase price: 50+20=70 yuan.

The purchase price is 70 ÷ (112.5%-1) = 560 yuan.