Thank u loads! Much appreciated :) understood clearly.

Am still grasping why we need to add a 29 when total strin is 576cm. How do i explain to a kid sorry

ahh i see! okay so basically since we know P and Q combined are already four units total (since P is thrice of Q) , hence in order to be able to find the value of one unit, we need to ensure R is also able to be a unit on its own. To do that , we simply add 29 to 576 in order to form that extra unit R (as shown in the model for R) to make up for R's lesser length :) by adding 29, we are making R the same length/unit as Q. that way we get 5 equal units to divide!

hope this isnt too lengthy

Thank u Clarice! Appreciated:)