1998 AHSME Problem 25

A piece of graph paper is folded once so that (0,2) is matched with (4,0), and (7,3) is matched with (m,n). Find m+n.

\mathrm{(A) \ }6.7 \qquad \mathrm{(B) \ }6.8 \qquad \mathrm{(C) \ }6.9 \qquad \mathrm{(D) \ }7.0 \qquad \mathrm{(E) \ }8.0

Full credit goes to MAA for authoring these problems. These problems were taken on the AOPS website.