Answer
Find out the what fraction of the sandcastle did they build together .
To prove
Let us assume that the fraction of the sandcastle they build together be x .
As given


Than the equation becomes

L.C .M of (3,7) = 21
solving


Therefore the fraction of the sandcastle did they build together be

Hence proved
C. I, II, and III
I’m sure it’s the answer not positive though, I’m sorry.
Answer:
990.36$
Step-by-step explanation:
You times 18 by 42 then you get 756 then you times that by 1.31$
8k-4k+4+7-k =
8k-4k-k+4+7 =
3k+11