73, because 10+7= 17, this is one time; then 17+7=24, this is two times; next you do 24+7=31, this is three times; next you do 31+7=38, this is four times; then you do 38+7=45, this is five times; then you do 45+7=52, this is six times; then you do 52+7=59, this is seven times; next you do 59+7=66, this is eight times; finally you do 66+7=73.
