Challenging P6 SA1 P2 Pei Hwa Q18

A marathon runner has some water in his water bottle. Every time he comes to a refill station, he doubles the amount of water in his bottle. Every time he comes to a rest stop, he drinks 500 ml of his water. During one race, the marathon runner passes a refill station and a rest stop three times. After the third rest stop, he finds his water bottle empty. How much water did he have in the bottle at the beginning of the race? [5]       

Work backwards, step by step to derive the amount of water at the beginning.

3^rd Stop    3^rd Refill    2^nd Stop     2^nd Refill    1^st Stop    1^st Refill

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  500           250         750             375          875         437.5

- 500       + 250       - 500          + 375       - 500          437.5

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    0            500         250              750         375          875

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He had 437.5 ml water in the bottle at the beginning of the race.