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Test series ques

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Johnny has bought two candles of equal length but different diameters. One of the candles will burn up completely in 10 hours, while the other candle requires 40% more time to burn up completely. If the candles are lit at the same time, approximately how long will they burn before one of the candles is twice the length of the other?

1)7.8 hours

2)6.4 hours

3)5.6 hours

4)6 hours
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suppose both candle of length x meter.

1 candle burning  x/10 meter per hour

2nd candle burn completely in 10*140/100=14 hour

so ,2nd candle burning x/14 meter/hour

let after y hour 1 st candle length is half of 2 nd candle as 1st candle burn faster

so , length of 1st candle after y hour=x-x/10*y

length of 2nd candle after y hour=x-x/14*y

so length of 1st candle should be half of 2nd candle

x-xy/10=1/2(x-xy/14)

x-xy/10=x/2-xy/28

x/2=18xy/280

y=280/36 = 7.777 hour  ~ 7.8 hour
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