A150 km trip was covered by a motorcycle going at an average speed of 75 km/h, by a bus doing 60 km/h, by a truck doing 50 km/h, and by a bicycle doing 20 km/h. chapter reference hint b are speed and time directly proportional? inversely proportional? what is the constant of variation?

By definition, the speed s and the time t and the distance d are related by the following rule:

We deduce that the speed and the time are inversely proportional. (meaning that while one parameter is increasing, the other parameter is decreasing)
The third parameter is the distance, which equate to 150 km as illustrated in the previously given example. 

B. because the 7 represents "seven times", x represents "a number", 12 represents "plus twelve", and -4 represents "minus four"