Final answer:
The grid ratio is found by dividing the height of the grid by the interspace width, which in this case is 6:1 for the given measurements of 120 mm and 20 mm, respectively.
Step-by-step explanation:
The student is asking about the grid ratio for an x-ray grid. The grid ratio is the ratio of the height of the grid to the width of the interspace. It can be calculated using the formula: Grid ratio = Height of the grid / Interspace width. Here, the height of the grid (H) is 120 mm and the interspace width (W) is 20 mm. So, the grid ratio would be 120 mm / 20 mm = 6. This means the grid has a ratio of 6:1.