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Closing speed in mixed units

Mental mathHard

$2$ trains approach each other on straight parallel tracks, one at $72$ km/h and the other at $54$ km/h. They are $1.05$ km apart. After how many seconds do they meet?

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Hint

Add the speeds and convert to m/s using a single factor.

Solution

The closing speed is $72 + 54 = 126$ km/h.
Divide by $3.6$ to convert to m/s: $126 \div 3.6 = 35$ m/s.
The gap is $1.05$ km, which is $1050$ m.
The time is $1050 \div 35 = 30$ , so they meet after $30$ seconds.

Answer: $30$

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