The down payment is 7800 yuan. If 10000 yuan is enough, you need to pay 8000 yuan.

So the down payment is less than 10000, and there is no discount, so the down payment is 7800 yuan.

The second payment of 26 100 is definitely more than 20,000 yuan. If 30,000 yuan is enough, at least 30,000 * 0.9 = 27,000 yuan must be paid, so the second payment is more than 20,000 yuan, and only 10% discount can be given for those below 30,000 yuan.

The second payment, before discount, is: 26100/0.9 = 29,000 yuan.

The total payment of these two times is 7800+29000=36800 yuan before the discount, and if it exceeds 30000 yuan, you can get a 20% discount on the excess.

To sum up, the original payment is made in two installments. There is no discount for the first time, and 10% discount for the second time.

If you buy the same amount of raw materials at one time, you will get a discount of 10% for the payment of 30,000 USD, and 8% for the rest.

That is to say, combined payment, the second payment remains unchanged, or you have to pay 26 100 yuan, of which 7800 yuan for the first time, some are 10% off, and some are 20% off.

The underpaid money is the part reduced after the first total payment of 7800 yuan.

Now look at this explanation:

When you pay in two installments, the second installment is 10% off. Just look at how much of the down payment of 7800 yuan is 10% off and how much is 20% off.

A * * * 30,000 yuan with a 10% discount, and 30,000 * 0.9 yuan is left after the discount.

After the second payment is 10% off, it will be 26 100 yuan.

Therefore, in the initial 7800 yuan, the discount of 10% will be:

30000*0.9-26 100 yuan

Before discount: (30000 * 0.9-26100)/0.9 =1000 yuan.

That is to say, of the 7800 yuan paid for the first time, 1000 yuan was 10% off, and 1000*0. 1 yuan was underpaid.

The remaining 7800- 1000=6800 yuan, with a 20% discount, and the underpayment of 6800*0.2 yuan.

1* * underpaid:1000 * 0.1+6800 * 0.2 =1460 yuan.

The bricklayer has a good idea, but it is not easy to understand if he omits too much analysis.

The conventional plan, just calculated, should be 36800 yuan before the two payments are discounted.

Consolidated payment:

10% discount for 30000, and 20% discount for the remaining 6800.

A * * * demand:

30000*0.9+6800*0.8=32440 yuan

Separate payment, one * * * paid:

7800+26 100=33900 yuan

Consolidated payment can be underpaid: 33900-32440= 1460 yuan.

If you don't understand, ask me. . .