What does this simple rebus mean?

In a fruit store, there was a unique weighing machine that was made to weigh only cherries and strawberries as they were priced the same.

Other fruits like watermelons or mango had different machines as they were expensive.

A man successfully buys watermelons at the price of cherries. How?

Twice ten are six of us,

Six are but three of us,

Nine are but four of us;

What can we possibly be?

Would you know more of us?

Twelve are but six of us,

Five are but four, do you see?

What are we?

Find the hidden tiger in the given optical illusion ?

A Scientist was fifty years old in the year 2000 but he was forty years old in the year 2010. How can this be possible?

PS: It has a logical answer.

We have four gears with below features=> Both Gear A and D has 60 teeth=> Gear B has 40 teeth=> Gear C has 20 teeth=> Every two minutes, Gear B makes 25 full turns.What is the relative speed of Gear A and Gear B?

I am a white box having no key or lid.
Yet there is a golden treasure inside it.

What Am I?

When 99 is considered higher than 100?

You have a three-gallon and a five-gallon measuring device. You wish to measure out four gallons.

You may enter, but you may not come in. I have space, but no room. I have keys, but open no lock. What am I?