Well it's even worse now. Apparently you can just repeatedly get 3 DCoins by solving a Brainiac Language notebook, right or wrong.
Really, I think what should happen is that Store prices should go up, you should get more DCoins if you solve questions right rather than just skip through the notebooks, and all the DCoins you got from notebooks should be multiplied if you beat the level.
Even so, you still have the option to grind away and be rewarded for wasting time. You see? It's a messed up reward system.