1。 Let the number of one digit be x, then the number of the tenth digit is 6-x, according to the meaning of the question:

10(6-x)+x+ 18 = 10x+(6-x)

2. If the number of cantaloupes is x and the number of watermelons is 2x, it will take x/40 days to sell out cantaloupes.

2x-x/40×50=360

3. Let Qian Ming be X yuan, then Sean is 0.25x+60.

x+(0.25x+60)= 1280

How to solve the equation is up to you.