There were 12 peaches on the tree originally.
Let's make an equation to describe the problem and then solve it.
"Bill picked 1/4 of the peaches on his grandmothers tree."
Which means that 3/4ths of the original number of peaches remain. So:
(3/4)x
"After Bill finished Sally picked 1/3 of the peaches that were left on the tree."
Which means that she left behind 2/3rds of the peaches that Bill left behind. So:
(2/3)((3/4)x)
"After Sally finished there were 6 peaches left on the tree." So:
(2/3)((3/4)x) = 6
Now we just need to solve for x.
(2/3)((3/4)x) = 6
(3/2) * (2/3)((3/4)x) = 6 * (3/2)
(3/4)x = 9
(4/3)(3/4)x = 9*(4/3)
x = 12
There were 12 peaches on the tree originally.