A sixty-four dollar question (also written $64 question) is the critical question about a problem or a crucial issue, something difficult if not impossible to answer.
This comes from
a popular 1940s U.S. radio quiz show, Take It Or Leave It, which offered $64 as the largest prize. The first question had a prize of $1 and the prize total doubled with each successive question: $2, 4, 8, 16, 32, culminating in the $64 question. (Like Who Wants To Be A Millionaire if they actually stuck to the same ratio between questions.) There was later a CBS television quiz show in the U.S., The $64,000 Question, based on the radio show. There the first question was the $64 dollar one, and the contestants then worked their way up to the top prize, $64,000. It and its first spin-off, The 64,000 Challenge, were hosted by Hal March. A later, syndicated spinoff was "The $128,000 Question."
Because of this change between the radio and the TV versions, as well as inflation, the expression is also heard as "sixty-four thousand dollar question" and even "sixty-four million dollar question," (as well as the numeric representations of those amounts) and has led to such take-offs as "sixty-four cent question," "sixty-four franc question" and "sixty-four bit question" (the latter referring specifically to using 64-bit technology).
A vocal version of the TV show's instrumental theme music, sung by Hal March, was released as a 45 rpm single called "Love is the Sixty-Four Thousand Dollar Question" in 1956. Its lyrics were by Fred Ebb, and the music by Norman F. Leyden. A songwriter called Dave Keats has written a different song called "Sixty-Four Dollar Question."