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  4. The traffic lights at three different road crossings change after every 54 sec .63 sec. and 81 sec. respectively. After how many seconds the light will change simultaneously.

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Q. The traffic lights at three different road crossings change after every $54 sec .63 sec$. and $81 sec$. respectively.
After how many seconds the light will change simultaneously.

Real Numbers

Solution:

Interval of change $=($ L.C.M of $54,63,81) \mathrm{sec}$ $=3 \times 3 \times 3 \times 3 \times 2 \times 7=1134 \mathrm{sec}$.
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So, the lights will again change simultaneously after every 1134 seconds i.e. $18 \mathrm{~min}$. $54 \mathrm{sec}$.