This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.
Q: Some clock-shift problems could be avoided by adjusting clocks continuously or at least more graduallyfor example, Willett at first suggested weekly 20-minute transitionsbut this would add complexity and has never been implemented.
A:
Willett's first suggestion was to change clocks by 20 minutes how often?