Answer:

The number of students in third grade 45
Step-by-step explanation:
Given : There are 92 students in the third and fourth grades at Johnsonville Elementary.
Of these, 47 are in fourth grade.
To Find : Write an equation to find how many third graders there are. Use a question mark to represent the unknown number and solve.
Solution:
Number of students in fourth grade = 47
Let ? be the number of students in third grade
So, total number of students = 
We are given that There are 92 students in the third and fourth grades
So, 


Hence an equation to find how many third graders there are is
and the number of students in third grade is 45.