Easiest way is to use formula give in Traffic Engineering Handbook, Pg: 332
L = VK25(1+p)/Nc
v= peak 15-min flow rate
K = constant to reflact random arrival of vehicle, usually 2
p = percent trucks
Nc = no. of cycle per hour
= 300*2*25 (1+0.1)/40
Answer is B = 413 ft