How many integer nonnegative solutions are there to the pair of equations: $ x_1 + x_2 + x_3...+ x_6 = 20 $ and $ x_1 + x_2 + x_3 = 7$.
How many nonnegative integer solutions are there to the pair of equations $x_1+x_2+…+x_6=20$ and $x_1+x_2+x_3=7$?