Jocelyn borrowed 3.75kg which is equivalent to 3750grams
Assuming a batch is one cookie recipe then:
3 batches * 225 grams = 675grams
Jocelyn will use 675 grams of flour to bake 3 batches of cookies.
A cake recipe requires 1.2kg (equivalent to 1200grams).
She wants to bake 2 cakes
2 cakes *1200grams = 2400 grams
Jocelyn will use 2400 grams of flour to bake 2 cakes.
Add the total amount of flour she is going to use
2400 (for the cake) + 675grams (cookies) = 3075
She will use 3075 grams of flour in total.
Now take original borrowed amount from the amount she will use
3750-3075= 675
Therefore, Jocelyn will return 675 grams of flour back to her grandmother
But obviously we must assume she doesn't spill any flour when she makes the cake but in this situation it doesn't ask for assumptions. So the answer should be 675 grams of flour.
