Sam is participating in a 4 - day cross-country biking challenge. He biked for 57, 47, and 47 miles on the first three days. How many miles does he need to bike on the last day so that his average (mean) is 55 miles per day?

(57 + 47 + 47 + x) : 4 = 55

multiply both sides by 4 and you get

57 + 47 + 47 + x = 55 (4)

combine like terms on the left side of the = sign and multiply on the right side of the = sign to get

151 + x = 220

now just subtract 151 from both sides to isolate 'x'

x = 69