a)
s(1800) / 1800 = 6150/1800 = 41/12 = 3.417 m/s
b)
s1'(t) = 1/120·t
s2'(t) = 5
s3'(t) = 8 - 1/300·t
s1'(t) = 1/120·t = 2 --> t = 240 s
s3'(t) = 8 - 1/300·t = 2 --> t = 1800 s
s1'(t) = 1/120·t = 4 --> t = 480 s
s3'(t) = 8 - 1/300·t = 4 --> t = 1200 s
s(t) = 5 --> t ∈ [600, 900]
c)
s1'(t) = 1/120·t = 41/12 --> t = 410 s
s3'(t) = 8 - 1/300·t = 41/12 --> t = 1375 s