How do i solve the following questions?
1.How many nonnegative solutions are there to the pair of equations
x1 + x2 + x3 = 6, x1 + x2 + x3 + x4 + x5 = 15?
2. There is a train station in Smalltown where there are trains going north and trains going south. If there are 3 southbound trains and 4 northbound train departures in a day, how many ways are there to leave Smalltown by train?