Is the answer clear to you? This is a simple riddle it seems, but the answer is not so simple. Many people said the answer is 22, but, the obvious answer is not the right one always.
Still, think. There are just 2 people at the picnic, the couple. But, read again. Nobody says the young people went with them too. The first sentence has the answer. Most people oversee this somehow.
Brain teasers like this are very clear and obvious, only if you are focused. Do not feel bad if you gave a wrong answer. Just keep the focus sharp and you will get the drift. Optical riddles are working in a similar way. Next time you solve a riddle, you will give the right answer right away.
If you still seek some more of these challenges, you can try harder math solving riddle or similar. If you are right on the first try, congrats. Your mind is very sharp!