Question 142851
Suppose, Edmund's speed is 'v' mph.
By that time Lucy rode for 2 hrs. (9 pm to 11 pm), Edmund rode for ( 2 + 3 =) 5 hrs. (since he started 3 hrs. early).


By 11 pm Lucy had rode a distance {{{S[Lu] = 2 x 20 = 40}}} mph. 
By 11 pm Edmund had rode a distance {{{S[Ed] = 5 x v = 5v}}} mph. 


Since, still Lucy is 40 miles behind then, 
Distance covered by Lucy + 40 miles = Distance covered by Edmund
40 + 40 = 5v
5v = 80
v = 80/5 = 16


Hence, Edmund rode at the speed 16 mph.