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Answer :
The minimum distance that each boy should walk must be the least common multiple (LCM) of the measure of their steps.
$$
\begin{array}{|l}
\llap{7~~~~} 63, 70, 77 \\ \hline
9, 10, 11
\end{array}
$$
LCM = 7×9 ×10× 11 = 6930 cm
Hence the minimum distance is 6930 cm .