Solution: The speed of old freight train is 0.5 miles per minute and the speed of new dart express train is 1.5 miles per minute.
Explanation:
Let the speed of old freight train is x miles per minute and the speed of new dart express train is y miles per minute.
It is given that the speed of the dart express train is three times faster than the freight train. So, it can be written as,
....(1)
The trains are moving towards the opposite directions, therefore the distance between them is increased at the speed of
miles per minute.

The distance between both train in 15 minutes is represented by
.
According to the given information the distance between trains is 30 miles in 15 minutes.


Use equation (1) and put 



Put this value in equation (1), to find the value of y.


Therefore, the The speed of old freight train is 0.5 miles per minute and the speed of new dart express train is 1.5 miles per minute.
Change pounds to ounces.
1 pound = 16 ounces.
4 x 16 = 64 + 6 = 70 total ounces of clay.
Now divide by 10:
70 / 10 = 7
Each part was 7 ounces.
The cost of 5 apples is $6
To answer the problem above, use the concept of ratio and proportion. The ratio between 1 cm to 21 km should be the same as the ratio of 3.1 cm to the actual distance. Let x be the unknown actual distance and mathematically express the situation,
(1 cm/ 21 km) = (3.1 cm / x km)
This equation gives a value for x which is 65.1 km. Therefore, the answer is letter C.