Answer:
10t + D = 40
Explanation:
y = mx + b is the equation of a line in slope intercept form. We'll be using this equation since the slope, or distance traveled, is steady (linear). In this case, the slope is 10 since Sarah's pace is 10 km/hr. m is the slope, so m = 10.
y = 10x + b. However, she needs to run 40 km. So we're going to substitute 40 in for y.
40 = 10x + b. Now, we need to use t instead of x since that's what's been asked of us. (The slope represents the distance traveled over time, so t fits.)
10t + b = 40
b needs to become D, the distance in kilometers.
So now we have 10t + D = 40