With the development of the national economy, cars have entered the family. A car runs 100km on a flat road at a certain speed and consumes 10L of gasoline. If 30% of the heat released from the complete combustion of these gasoline is used to drive the car to do useful work, the calorific value of the gasoline used is 4.6 × 107j / kg, and the density is 0.7 × 103kg / m3 The resistance of the car at that speed

With the development of the national economy, cars have entered the family. A car runs 100km on a flat road at a certain speed and consumes 10L of gasoline. If 30% of the heat released from the complete combustion of these gasoline is used to drive the car to do useful work, the calorific value of the gasoline used is 4.6 × 107j / kg, and the density is 0.7 × 103kg / m3 The resistance of the car at that speed

(1) ∵ ρ = MV, v = 10L = 10 × 10-3m3, ∵ M = ρ, v = 0.7 × 103kg / m3 × 10 × 10-3m3 = 7kg, heat released by complete combustion of gasoline: Q discharge = MQ = 7kg × 4.6 × 107j / kg = 3.22 × l08j; (2) according to the title, the useful work used to drive automobile: w have = q discharge × 30% = 3.22 × l08j × 30% = 9

A truck travels 50 kilometers per hour. Two hours after starting, a car starts from the same place at a speed of 90 kilometers per hour and goes in the same direction as the truck,
The car can catch up with the truck after a few hours of departure,

Let's catch up in X hours
（90-50）x=50*2
40x=100
x=2.5
If you don't understand this question, you can ask,

The speed of a car is 1.4 times that of a truck. The two cars start from 592.8 kilometers apart and run in opposite directions. How many hours can they meet?

Let's meet in X hours
38x+1.4×38x=592.8
91.2x=592.8
x=6.5
That is, they can meet at 6.5 hours

A passenger car and a freight car travel from two places 900 kilometers apart at the same time. After 6 hours, they meet. The speed ratio of the two cars is 2:3. How many kilometers does the passenger car travel per hour?
Another way
There are 350 fruit trees in the orchard, of which the ratio of apple trees to peach trees is 2: the ratio of peach trees to pear trees is 4:5. How many of these three kinds of fruit trees are there? Don't forget this!

Speed sum of two cars = 900 / 6 = 150 km / h
Bus speed = 150 * (2 / 5) = 60 km / h
The bus runs 60 kilometers per hour
Apple tree = 2 / 3 peach tree
Pear = 5 / 4 peach
Peach tree = 350 / (1 + 2 / 3 + 5 / 4) = 120
Apple trees = 120 * 2 / 3 = 80
Pear tree = 120 * 5 / 4 = 150