Solution:

If you read the question carefully, you will find that both times are same.
An hour and a half are equal to 90 minutes only.

Solution:

No stairs, Its a one story building.

Solution:

Explanation:

Since Rohini is 21st from either side, there must be 20 people in front of her and 20 people behind her.
20 + 20 + 1 (she herself) = 41
Therefore, the queue consists of 41 people.

Solution:

295 minutes.

Explanation
When he cut 59 stocks, the 60th stock will remain.
59 * 5 = 295 minutes.

Solution:

You are the ship captain. Read the first sentence again. Therefore, the color is same as your eyes color.

Solution:

Alexandra used two planks as shown in the picture below.

Solution:

You should reject the bet. When you give him $100, he won't be able to convert it into $500 dollar and will lose the bet. Thus, he'll give you $50. But your $100 will be with him and therefore, you are going to lose your money anyhow

Solution:

It is an easy task. You just have to give the last chocolate along with the basket. That way, all gets a chocolate and the last one remains in the basket as well.

Solution:

The fastest or even the slowest rabbit can only run till the mid point into the given garden. After the mid point, it will actually be running out of the given garden.

Solution:

The timer of a microwave.

The timer of a microwave generally consists of 4 digits. First two digit for minutes and next two for seconds.
So,
99 indicates - 1 minute 39 seconds
100 indicates - 1 minutes (1:00)