For an instance, n represents the number
The equation model will be
Solve the equation
move 3 to the right side, division by 3 will be multiplication by 3 after being moved to the right side

n + 5 = 21
move 5 to the right side, addition by 5 will be substraction by 5 after being moved to the right side
n = 21 - 5
n = 16
The number described is 16