David took 6 hours to travel from Country Z to Country A at 60 km/h. Then he travelled from Country A to another country at 1.8 times his original speed. 40% of the distance from Country Z to Country A is equal to ¹₃ of the distance from Country A to the next country. Find the average speed for the whole journey and express it as a mixed number.
Distance he travelled from Country Z to Country A
= 6 x 60
= 360 km

Speed from Country A to another country
= 1.8 x 60
= 108 km/h

40% = ⁴⁰₁₀₀ = ²₆

²₅ Distance from Country Z to Country A = ¹₃ Distance from Country A to next country
²₅ Distance from Country Z to Country A = ²₆ Distance from Country A to next country

5 u = 360
1 u = 360 ÷ 5 = 72

Distance from Country A to the next country
= 6 u
= 6 x 72
= 432 km

Time taken from Country A to the next country
= 432 ÷ 108
= 4 h

Total distance of the whole journey
= 360 + 432
= 792 km

Total time taken for the whole journey = 6 + 4 = 10 h

Average speed of the whole journey
= 792 ÷ 10
= 79 ²₁₀
= 79 ¹₅ km/h

Answer(s): 79 ¹₅ km/h