28. An image is partitioned into 2 regions-one white and the other black. A reading taken from a randomly chosen point in the white section will give a reading that is normally distributed with µ = 4 and σ² = 4, whereas one taken from a randomly chosen point in the black region will have a normally distributed reading with parameters (6, 9). A point is randomly chosen on the image and has a reading of 5. If the fraction of the image that is black is

a, for what value of a would the probability of making an error be the same whether one concluded the point was in the black region or in the white region?