A bag contains $5$ white and $7$ black balls and a man draws $4$ balls at random. The odds against these being all black is :

A trader marked a watch $40\%$ above the cost price and then gave a discount of $10\%$. He made a net profit of $Rs.468$ after paying a tax of $10\%$ on the gross profit. What is the cost price of the watch?

$42$ men take $25$ days to dig a pond. If the pond would have to be dug in $14$ days, then what is the number of men to be employed?

If the diameter of a wire is decreased by $10\%$, by how much percent (approximately) will the length be increased to keep the volume constant?

From a series of $50$ observations, an observation with value $45$ is dropped but the mean remains the same. What was the mean of $50$ observations?

Mr. Duggal invested $Rs.20,000$ with rate of interest $@ 20\% p.a$. The interest was compounded half-yearly for the first one year and in the next year it was compounded yearly. What will be the total interest earned at the end of two years?