The measure off DF is 11.
In a circle inscribed within a triangle, the distance from each vertex of the triangle to the two nearest touchpoints (points of tangency on the circle) are equal. Since SD=4, DT=4 as well. Since UF=7, then FT=7.
DF=DT+TF=4+7=11.
(4 hundred + 0 tens + 5 ones) + (1 ten) = 4 hundred + 1 ten + 5 ones
... = 415
