Mrs. cox is baking a ham for dinner. it takes 1 hour 30 minuets to bake . the family eats at 6: 15 p. m. what time should mrs. cox put the ham in the oven?

Mathematically: 

It takes 1 1/2 hours to bake, and the family eats at 6:15 pm. 
Subtract 1 hour and 30 minutes from 6:15, to get 4:45 pm

Mrs. Cox should start baking at ~4:45 pm.

However, due to preperations, she should actually "start" baking at 4:30 pm or even earlier

hope this helps