2. It is known that X3m-2n+ 1+9 = 3m- 1 is a linear equation about x, so the conditions that m and n should meet are:

3. Someone bought 20 stamps with 60 cents and 80 cents, spent 20 cents on 13 yuan, and then bought stamps with 60 cents.

I bought one, and I bought an 80-cent stamp.

There are two kinds of alcohol, one is 60% and the other is 90%. Now we need to prepare 300 grams of essence with 70% concentration. Q: How many grams do you need to take? (200g,100g)

It takes five days for Party A to do a job alone, three days for Party B to do it alone, and one day for both of them.

6. If there is a two-digit number, the difference between one digit and ten digits is 6, and one digit is twice as big as ten digits, then this two-digit number is.

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