If 5 cats catch 5 mice in 5 minutes, how long will it take one cat to catch a mouse?

Can you solve the below tricky visual equation?

I welcome the day with a show of light, I stealthily came here in the night. I bathe the earthy stuff at dawn, But by noon, alas! I'm gone.

I have Rs.50 and I spend it in the way as below.

The person who makes it has no need for it; the person who buys it has no use for it. The person who uses it can neither see nor feel it. What is it?

How many Watermelons are there in the below picture?

Irene killed hundreds of people mercilessly and yet the police were not able to put her behind the bars. Why?

Known by all without exception, Forever here, for your protection, Sometimes strong, sometimes weak, Right after the night I come - hot and chic. And while millions of miles away, I always get to you, I find my way. No life around could do without me, Can you guess what I might be?

The answer I give is yes, but what I mean is no. What was the question?

Can you solve this amazing hard number series problem?
7, 8, 10, 12, 16, 18 ?