Why would our teacher make us guess.
I got it
She would need to teleport back to be on time for piano lessons
let me explain
If she needs to go average 4 mph for 8 miles, she has 2 hours
If she only goes 2 MPH on the way over, and its 4 miles each way, it took 2 hours to get to grandma's.
Therefore, by the time she gets there, she is already late.