At 7 a.m., Justin started from City Y and travelled towards City Z and his speed remained constant throughout. At 10 a.m., Erika started her journey from City Y towards City Z at an average speed of 72 km/h. Erika overtook Justin at 1 p.m. After overtaking, Erika carried on her journey at the same average speed and reached at City Z at 6:35 p.m.

1. Find Justin's average speed in km/h.
2. What is the distance between the two cities?

(a)
From 10 a.m. to 1 p.m. = 3 h

Distance that Erika had travelled until Erika overtook Justin
= 3 x 72
= 216 km

From 7 a.m. to 1 p.m. = 6 h

Justin's average speed
= 216 ÷ 6
= 36 km/h

(b)
From 10 a.m. to 6:35 p.m. = 8 h 35 min

1 h = 60 min
8 h 35 min = 8 ³⁵ ₆₀
 h = 8 ⁷₁₂ h

Distance between the two cities
= 8 ⁷₁₂ x 72
= ¹⁰³₁₂ x 72
= 618 km

Answer(s): (a) 36 km/h; (b) 618 km