How can I know If I use interpolation or extrapolation after I estimated the volume of 3 marbles?

Answer 1

interpolation and extrapolation in grade 4 ??? WOW

since the relation is linear, it makes no difference whether what you call it.
you are probably just going to use a simple ratio.

Your question is probably something like this, "If 3 marbles weigh 15 g, how many
do 2 weigh ?
do 8 weigh ?

in either case: 15/3 = x/whatever number