It can be any number. That is why mathematicians consider it undefined. 0/0 comes up mostly in calculus as the derivative. The derivative is defined as an instantaneous rate of change. Or a slope found at one point. Remember that slope is defined as the change in y divided change in x. But, you you want the exact slope at one exact point, you have zero change in y and zero change in x. That would be 0/0. However, we can think of an infinite number of lines with an infinite slopes. But all one fraction. To deal with this issue, we have to use limits and approach the value carefully.