Combinatorics problem! Decent amount of points How many 4-digit decimal numbers have at least a 7, an 8, and an 9? Not an 8,9, or 7, the 4 digit number must contain all 3 in it's digits. Some examples of such numbers are: 9871, 8279, and 5798. Repost cause of trolls...

Johnny rode his bike to a friends house 4 blocks down the street in his neighborhood. he immediately rode back home once he realized his friend was unable to play. what was his displacement for the total bike ride trip? what could you use as a reference point ? show the steps to solve this problem.