Exercise 2 solution (Python)

def to_min(t):
    h, m = t.split(":")
    return int(h) * 60 + int(m)

def to_clock(mins):
    return f"{mins // 60:02d}:{mins % 60:02d}"

def naismith(dist_km, ascent_m):
    return round(dist_km * 12 + ascent_m / 100 * 10)

def plan(start, legs, stops, dark, margin=60):
    walking = sum(naismith(d, a) for d, a in legs)
    finish = to_min(start) + walking + stops
    latest = to_min(dark) - margin
    status = "OK" if finish <= latest else "TOO LONG"
    return walking, to_clock(finish), status

legs = [(3.0, 200), (4.0, 400), (5.0, 0)]
print(plan("09:00", legs, 45, "16:30"))
print(plan("13:00", legs, 45, "16:30"))

Output:
(204, '13:09', 'OK')
(204, '17:09', 'TOO LONG')

The legs take 56 + 88 + 60 = 204 minutes of walking. Starting at 09:00 with 45 minutes of stops you finish at 13:09, well before the latest safe time of 15:30 (dark at 16:30 less a 60 minute margin). Starting at 13:00 you would finish at 17:09, after dark, so the plan is rejected at the table, not discovered on the hill.