If  the distance between two houses is 2u, at Town B, Louis travelled u+180. Clark travelled u-180. Louis travelled 360km more. 

Part (a) is to help the child solve part (b)

                              Clark   :     Diff     : Louis 

Actual speed         60     :       24      :     84

Ratio speed            5       :       2        :      7

Ratio distance       5       :       2        :       7

Actual distance.  *900  :  360 

* 2 units = 360

1 unit = 180

5 units = 900km

thanks

You are welcome. Just another note.

Alternatively more common method might be to consider Louis’s travel:

extra distance/extra speed = 360/24 = 15 hrs

Both travelled for 15 hours each.

Clark has travelled S x T = 60 x 15 = 900km#

(distance of Clark’s house from Town B)

distance between :
Town A and Lois’ house/Clark’s ——- 1u
Town B and Clarke’s house ——- 1u – 180
Town B and Lois’ house ——- 1u +180
1u +180 – (1u – 180) = 360

(b)
60 + 24 = 84 (Lois’  speed)
60/(1u – 180) = 84 /(1u + 180
60u + 10800 = 84u – 15120
84u – 60u = 24u ——- 10800 + 15120 = 25920
1u ——- 25920/24 = 1080
1u – 180 ——- 1080 = 900

Ans : (a) 360km; (b) 900 km.

thanks