Suppose an empty bottle weighs x grams and water weighs y grams.

According to the meaning of the question, x+2y=450 and x+5y=750.

X=250, y= 100 If two equations are solved simultaneously.

the second question

1÷7= 1/7, daily average maintenance 1/7.

8÷7=7/8, that is, 7/8 kilometers are repaired every day on average.

Third question

12 ÷19 = 12/ 19, that is, each skirt needs12/19 meters.

1÷19 =119, that is, each skirt119.