To analyze the effect of two categorical variables (presence of people in the room and whether the television is on or not) on a continuous variable (amount of food eaten by the guinea pig), Jack can use a two-way ANOVA (analysis of variance) test.
ANOVA is a statistical test used to compare the means of more than two groups. In this case, there are four groups (people in the room/TV on, people in the room/TV off, no people in the room/TV on, no people in the room/TV off).
A two-way ANOVA will allow Jack to determine whether there is a significant difference in the mean amount of food eaten by the guinea pig across the four groups, as well as whether the two categorical variables interact to affect the amount of food eaten.
If a significant difference is found, Jack can then use post-hoc tests (such as Tukey's HSD or Bonferroni) to determine which specific groups differ from each other.