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
Answer: it's -1
Step-by-step explanation: Hope this helps
Can I have brainliest?
The answer you are looking for is -17/44
3.12 = 3 4/33 (<span>3.12 repeating)</span>
hope it helps