SOLUTION: fill in the blank with one of the digits 2, 5, 10, 50 (using each digit only once) to make a true statement. using the following functions x-x/x+x=50

Let's use a different letter for all of them instead of 
the same letter x

A - B/C + D = 50

Since the other 3 numbers are so much smaller than 50,
either A = 50 and -B/C + D = 0 or D = 50 and A - B/C = 0

If A = 50, then we have to make 0 out of -B/C + D using 2, 5, and 10

we can have -10/2 + 5 = 0 or -10/5 + 2 = 0, so we have these two
solutions:

50 - 10/2 + 5 = 50
50 - 10/5 + 2 = 50

Of course we can reverse the first and last terms: 

5 - 10/2 + 50 = 50
2 - 10/5 + 50 = 50

So there are four solutions, or just two if you don't count swapping
the first and last terms.

Edwin