First, find out how much Jack and Jill own together:

1/3 + 1/4 =  ; the least common multiple of 3 and 4 is 12, so to get a common denominator for the fractions 

                      multiply both the numerator and denominator of the:

                               - 1st fraction by 4,

                                        and

                               - 2nd fraction by 3:

4/12 + 3/12 =  ; now that the fractions have a common denominator, add the numerators and keep the

                          denominator the same:

7/12 is the fraction that Jack and Jill own together.

Subtract that fraction from 1 to find out how much Bob owns:

1 - 7/12 =  ; 1 is also equal to 12/12:

12/12 - 7/12 = 5/12 (this is the part that Bob owns, and the answer to the problem).