Answer:
17
Explanation:
If tom had 347 marbles and traded 28 of them, then the number of marbles he would have would be the difference of what he had and what he traded (assuming he got nothing back or he lost the marbles). This will be
= 347 - 28
= 319
However, he trade them for some marbles he like and ended up having 336 marbles. It means that the number of marbles he got from trading 28 marbles would be the difference of what he had if he had just lost the 28 marbles and what he had after trading the 28 marbles.
The number of marbles he got from the trade
= 336 - 319
= 17