Honestly these are 2 seperate questions but they look the same to me. Isnt non negative numbers, positive numbers? I dont even no how to go abouts solving this

How many non negative integer solutions does the equation x+y+z+w=15 have?

How many positive integer solutions does the equation x+y+z+w=15 have?

( hint: define new variables x'=x-1, y'=y-1, z'=z-1, w'=w'1)