Rosa’s winning distance can be described as 1.5M, where M represents the mean of the three best practice distances from the table. Explain what the coefficient 1.5 represents in the context of the story.

The coefficient 1.5 means her winning distance is 1.5 times the mean (add them up and divide the total number by the number of numbers) of her three best practices.

Some other helpful stuff:

Let's say Rosa's three practice distances were 1.6 meters, 1.7 meters, and 2.1 meters.  To get the mean of these, add them up(5.4) and divide by the number of numbers, three(1.8). Let's say her winning distance was 2.7. If you divide the winning time by the mean of her three best practices, you get 1.5. This means that 2.7 = 1.5(1.8), or, 2.7  = 1.5M.

if you mean

then it's c. 8, -7/3

but if you mean

then it is