Arleen has a gift card for a local lawn and garden store. She uses the gift card to rent a tiller for 3 days. It costs $47 per day to rent the tiller. She also buys a rake for $8.

Okay, so y = kx where k is the constant of proportionality. 
Just plug in the values...the time affects the number of tables, so it must be the independent variable, or x value. 

5 = 15k

Divide both sides by 15, and you are left with 1/3. This is the constant of proportionality.