This number riddle asks: What can you put between 7 and 8 so the result is more than 7 but less than 8? It sounds as though you need to add another number, but the solution is much simpler.
The answer is: a decimal point.
Place a decimal point between the 7 and the 8 and you get 7.8. That number is greater than 7 and less than 8, exactly as the riddle requires.
The trick is that the question asks what you can “put between” the numbers. It never says the thing you insert has to be another number.
This is a good example of a puzzle that depends more on reading the wording carefully than on doing difficult math. Once you think of punctuation instead of arithmetic, the answer becomes obvious.
