1, two thirds of the number A and one quarter of the number B are equal (both numbers A and B are not 0), and A: I = (3): (8).

2. The former item of a proportion is reduced to one-third of the original, and the latter item is reduced to one-sixth of the original, and the proportion is four-fifths. This ratio is (2/5).

3、 15÷[ 1- 1/5-3/4×( 1- 1/5)]= 25(m)

4,200 ÷ (3/5+13/20-1) = 800 (vehicle)

Suppose: this barrel of gasoline has x kilograms.

x-3/5x+6= 1/2x

2/5x+6= 1/2x

1/2x-2/5x=6

1/ 10x=6

x=60

6. Design: This batch of cement was originally x tons.

2/5x+4.5 = 1/( 1+ 1)x

2/5x+4.5= 1/2x

1/2x-2/5x=4.5

1/ 10x=4.5

x=45