Answer
The average speed of Juno is $8927mi/hr$.
Work Step by Step
The closest distance between Earth and Jupiter is $391$ million miles, which is $3.91\times10^8mi$
The time it takes for Juno to reach Jupiter is $5$ years, which is $43800hr=4.38\times10^4hr$ (hours).
So, the average speed of Juno is $$v=\frac{3.91\times10^8mi}{4.38\times10^4hr}\approx8.927\times10^3mi/hr\approx8927mi/hr$$
