Question

Julie drives 100 mi to Grandmother's house. On the way to Grandmother's, Julie drives half the distance at 31.0mph and half the distance at 70.0mph . On her return trip, she drives half the time at 31.0mph and half the time at 70.0mph .
I get how to do the first part but how do you do it when she drives half the time at 31mph and half the time at 70mph?