I think that the first missing number is 25 because it is in the descending trend of the numbers: 400, 100, ?, 50, which is governed by a reduction of a factor 4. That is, 100 is 400/4, so ?=100/4=25.
Following the first missing number, now believed to be 25, we are in an ascending trend, where doubling is the factor, so 25 doubled is 50 and 50 doubled is 100. If the ascending trend were to continue, the second missing number would be 200; but if the missing number is part of a descending trend, then it, too, must be 25, and the start of another ascending trend, so that the last number would be the given value of 50. So we have the choice: is the second missing number 200 or 25? The question asks for the missing term—just one number—so if the two missing terms are one and the same value both of them must be 25.