The cumulative distance the bird travels is 4.4 km.
The time of motion of the bird is calculated from the principle of relative velocity.
distance covered by bird - distance of runner = distance between them
vb_t - vr_t = d
(vb - vr)t = d
where;
(28.4 - 7.1)t = 3.3
21.3t = 3.3
t = 3.3/21.3
t = 0.155 hr
d = 0.155 hr x 28.4 km/h
d = 4.4 km
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