Ralph spent 5/9 of an hour mixing paints and 7/9 of an hour painting. how much time in all did ralph spend on his project?

the pronoun reference replaces a noun, but you have to make sure there's no room for confusion.

in the first sentence "her" could be referring to shely or to the sister. the answer there is "shelly called her sister and told her about the new baby."

in the second sentence "it" could be referring to the roof or the floor. the answer there is "since the roof leaks on the family room floor, we need to repair the roof."

they are all talking about chocolate (next time you post a question with "which" post the choices too)