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Its quite simple actually. It has been explained, just going to explain it another way though (open your Windows Calculator):
1. 45m = 3/4h = .75
2. 35s*0.75 = 26.25
3. So in 45m car A traveled 26.25 km.
4. Car B is 15s faster.
5. So when car A has traveled 26.25km, car B starts.
6. Next you can do trial/error.
7. 1h has passed. 26.25+35=61.25km traveled by car A
8. 50 km traveled by car B.
9. Then we see we need about 10 more km. So increase 2 quarter hours. Then 3. Thats when you realize car B has traveled more then car A at 1 3/4 hours.
This method is a bit more complicated though.
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