I don't understand this,so can you please explain more clearly. Where did you get the 1/5?

Question

1.The point P divides the interval joining A(-1,-2) to B(9,3) internally in the ratio 4:1. Find the coordinates of P.

Answer 1

A(-1,-2),P(x,y),B(9,3).

x-(-1) = 4(9-x)
x+1 = 36-4x
5x = 35
X = 7.
y-(-2) = 4(3-y)
y+2 = 12-4y
5y = 10
Y = 2.

P(7,2).