Bob is participating in a 6-day cross-country biking challenge. He biked for 57, 57, 70, 63, and 46 miles on the first five days. How many miles does he need to bike on the last day so that his average (mean) is 58 miles per day?

Solution:

      58 = (57 + 57 + 70 + 63 + 46 + x)/6

      58 * 6 = 57 + 57 + 70 + 63 + 46 + x

        348 = 293 + x

       x = 348 - 293 = 55 miles