My calculations give assuming the radius of Uranus is 1 (which we can do as we are working with ratios):
Originally Posted by emmaastronomy
Also you have an error in your arithmetic, your result should be: 2.9 magnitude (the difference between this and the above is the approx. 2 magnitudes from my having forgotten the planetary radii (the way that you have written your answere suggests you handled the powers of 10 incorrectly)).
Reality check: Maximum brightness of Saturn 0.7 Mag, maximum brightness of Uranus 5.5 Mag.