If you solve for y, in this case it's 15y = 5.
Dividing 15 by what number gets you 5? It's 3. Since you're multiplying y by 15, you substitute y with (1/3) and you can change it to this now, and it makes the expression correct. 15(1/3) does equal 5. 1/3rd of 15 is 5. y = 1/3rd
