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20 five letter words start with US, and five of them account for nearly everything you would guess.
A second U leads at about twenty percent, then A, H and N at fifteen each and E at ten.
USING, USERS, USAGE, USUAL and USHER are the entries in ordinary use, with USURY and USURP behind them.
The remaining thirteen are obsolete or borrowed forms that no puzzle is realistically going to ask for.
So a count of twenty comes down to five reliable answers and two more that depend on the word list behind the puzzle.
USUAL, USURY and USURP all put a second U in the third box, which is the largest single shape on the page at twenty percent.
Two U letters in the first three boxes is rare enough elsewhere that spotting it here effectively names the branch for you.
USING is the guess to start with, being much the most common entry and testing four letters the other four do not share.
If the I misses, USUAL confirms or eliminates the doubled U shape in a single turn.
Both share USE in sound but not in spelling, since USAGE puts an A in the third box. They sit on different branches.
It is a real verb meaning to seize power, and it shares the USU shape with USUAL, so it is a reasonable second guess.
US rarely opens an English root. Most of what survives is built on use or usury, which is why the page comes down to two families.