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sixty-four-dollar question

or $64 ques·tion

[siks-tee-fawr-dol-er kwes-chuh n, -fohr-]
noun
  1. the critical or basic question or problem: Whether the measure will get through Congress this session or not is the sixty-four-dollar question.

Origin of sixty-four-dollar question

from the fact that 64 dollars was the largest prize on a popular radio quiz show in the 1940s
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018