It should only take nine minutes to cook the egg. If you want to try to figure out of your own as how it is done in this short amount of time before seeing the answer, stop reading now. To start, flip both hourglasses over and put the egg in the water. When the four minute hourglass runs out, flip it back over immediately. When the seven minute hourglass runs out, flip that back over immediately too. One minute later, the four minute hourglass will run out again. At this point, flip the seven minute hourglass back over. The seven minute hourglass had only been running for a minute, so when it is flipped over again it will only run for a minute more before running out. When it does, exactly nine minutes will have passed, and the egg is done.

Answer to Game No 27:  Many Coins but No Dollar

A half dollar, a quarter, and four dimes.

Answer to Game No 28: A Way of Writing 100 Using Only Digit 9

99  99/99 = 100

Answer to Game No 29:  How Many Chocolates

The first boy will eat 50 chocolates in 15 seconds
The another boy eats half of the chocolate in double the time, so he eats 50 chocolates in 60 seconds, that is why he will eat only 12.5 chocolate in 15 seconds
Thus both the boys will eat 50+12.5=62.5 chocolates in 15 seconds.

Answer to Game No 30: How Much Weight Potatoes Lost

If 100 pounds of potatoes have 1% potato matter,
it means there is one pound of potato matter in 100 pound potatoes.

After the potatoes dehydrated somewhat, this one pound accounted for 2% of the total mass (with the remaining 98% being water).

One pound is 2% of 50 pounds,
so the total mass of the potatoes after sitting out in the sun will be 50 pounds.