Read this: Yale has had many financial supporters, but some stand out by the magnitude or timeliness of their contributions. Among those who have made large donations commemorated at the university are: Elihu Yale; Jeremiah Dummer; the Harkness family (Edward, Anna, and William); the Beinecke family (Edwin, Frederick, and Walter); John William Sterling; Payne Whitney; Joseph E. Sheffield, Paul Mellon, Charles B. G. Murphy and William K. Lanman. The Yale Class of 1954, led by Richard Gilder, donated $70 million in commemoration of their 50th reunion. Charles B. Johnson, a 1954 graduate of Yale College, pledged a $250 million gift in 2013 to support of the construction of two new residential colleges.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): How much didn't Charles B. Johnson pledge to Yale in 2013?
unanswerable