The stem-and-leaf plot shows the tens and the ones digits. So, in 78, the tens is a 7 (on the left), so go down to where the left side says 7.
In 78, the ones digit is 8 (on the right).
Now, count how many 8s are on the right side in the same row that has 7 on the left.
There are four 8s in that row.
So, 4 students scored a 78 on the test.
Answer: B. 4 students
