How many solutions are there to x_1+x_2+x_3+x_4=25 where x_n are all non-negative and x_1 is at most 5?

Not sure where to start

could u replace x_4 as x_4'+10 to get x_1+x_2+x_3+x_4'=20

Then get... 23C3 from combinations formula for repetition is not allowed and order is irrelevant