We need to use the law of sines
sin P sin Q sin R
-------- = ------- = --------
p q r
The capital letters are the angles and the lower case are the side lengths
We know sin P and sin R and p
sin P sin R
-------- = --------
p r
2/5 1/2
------ = --------
8 r
Using cross products
2/5 r = 1/2 (8)
2/5 r = 4
r = 4 * 5/2
r = 10
