4/16 or 1/4 because 1 + 1 + 2 is 4 & 3 + 5 + 8 is 16 so the fraction will be 4/16 but u can also divide both by 4’s which is 1/4.
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
It is.................x+y=5