The stem-and-leaf plot shows the number of cans of food collected by various students for a food drive. What was the greatest number of cans collected?

Look in the last row, and at the last item in that row.
The last row shows a stem of 5. The last item in that bottom row is 6, so the leaf here is 6.
A stem of 5 and a leaf of 6 combine to get the value 56 according to what the key shows. Therefore 56 is the largest value.