1 Last year's investment was 3/5 of this year's. Here, this year's investment is regarded as the unit "1", which includes the quantitative relationship: this year's investment multiplied by 3/5= last year's investment.

2. 7/ 10 of the car speed is equal to the speed of the big truck. This sentence takes (the speed of the car) as the unit "1", in which the quantitative relationship is (the speed of the car) multiplied by 7/ 10= (the speed of the big truck).

There are 72 boys in grade six, accounting for 3/5 of the total number of students in grade six. How many students are there in the sixth grade? The quantitative relationship in this question is: (the total number of people) times 3/5= (the number of boys)

Formula: (number of boys) (72) divided by 3/5; Suppose there are X students in grade six, and make an equation (3x/5 =72).