#### Answer

Over the last 500 m, Marcella should run at an average speed of 3.82 m/s

#### Work Step by Step

We can find the time required to run at an average speed of 4.00 m/s:
$v_{ave} = \frac{d}{t}$
$t = \frac{d}{v_{ave}} = \frac{1000~m}{4.00~m/s} = 250~s$
The run must be completed in 250 seconds.
We can find the time it takes to run 500 m at an average speed of 4.20 m/s:
$t = \frac{d}{v_{ave}} = \frac{500~m}{4.20~m/s} = 119~s$
The time remaining is $250~s-119~s = 131~s$
We can find the average speed over the last 500 m:
$v_{ave} = \frac{d}{t} = \frac{500~m}{131~s} = 3.82~m/s$
Over the last 500 m, Marcella should run at an average speed of 3.82 m/s