Frank ran 65 km on Monday, 68 km on Tuesday, 66 km on Wednesday, and 80 km on Thursday. After doing another run on Friday he calculated his mean distance run per day to be 70 km. How far did Frank run on Friday?
it is 71
[ 65 + 68 + 66 + 80 + R ] / 5 = 70 where R = Friday's run
[ 279 + R ] / 5 = 70 multiply both sides by 5
279 + R = 350 subtract 279 from both sides
71 km = R