How do you solve (10-5r)^2 using fctoring

(10-5r)^2=(10-5r)(10-5r) or 
25r^2-100r+100
divide the whole thing by 5 and get
5(5r^2-20r+20)
divide the whole thing by 5 again
25(r^2-4r+4)
factor by finding what two numbers add to get -4 and multiply to get 4 (-2 and -2 work)
r^2-4r+4=(r-2)(r-2) or 25((r-2)^2)