To find the mAs at 30 inches when the mAs at 60 inches is 30 mAs, you can use the inverse square law. The inverse square law states that the intensity of a light source is proportional to the inverse square of the distance from the source.
Let's call the mAs at 30 inches "mAs2". Then, we can write the equation:
mAs2 = (mAs1 * (60^2)) / (30^2)
where mAs1 is the mAs at 60 inches, which is 30 mAs.
Plugging in the values, we get:
mAs2 = (30 * (60^2)) / (30^2)
mAs2 = (30 * 3600) / 900
mAs2 = 120 mAs
So, the mAs at 30 inches is 120 mAs.