Step-by-step explanation:
1. 'x' is the total no. of students and 'y' is no. of 6th graders.
1/3 of the total students are 6th graders.
So the equation will be
1/3 × x = y
x/3 = y
2. If x = 90
y = 90/3
y = 30
3. If x = 27,
y = 27/3
y = 9
4. If y = 42
x/3 = 42
x = 42 x 3
x = 126