If her normal size is 9 and her rock-climbing size is 42, then the factor is 42/9 or 14/3. So to convert any normal size to rock-climbing size, I just multiply by 14/3. Therefore, Adina's rock-climbing size is 6 times 14/3 or 28. 
Adina and Lynn are going rock-climbing and need to rent special shoes. Lynn is normally size 9 and knows that her rock-climbing shoe-size is 42. If Adina is normally size 6, what size rock-climbing shoes should she rent assuming shoe size is directly proportional to rock-climbing shoe size?