If all Goats are Sheep and all Sheep are Deers, are all Deers Goats ?

Can you make the below equation true by using any three numbers from "1, 3, 5, 7, 9, 11, 13 and 15"?

( ) + ( ) + ( ) = 30

Take away the whole and some still remain. What is it?

Spot 5 differences in two Christmas tree below:

It walks on 4 when its young, walks on 2 when it's older, and walks on 3 when its very old?

How many times can you subtract the number 5 from 25?

What number divided by 4 is the same as that number minus 4?

Find the value of "a" by solving the maths equation in the picture below:

What has 13 hearts but no other organs?

The host of a game show offers the guest a choice of three doors. Behind one is an expensive car, but behind the other two are goats.
After you have chosen one door, he reveals one of the other two doors behind which is a goat (he wouldn't reveal a car).

Now he gives you the chance to switch to the other unrevealed door or stay at your initial choice. You will then get what is behind that door.

You cannot hear the goats from behind the doors, or in any way know which door has the prize.

Should you stay, or switch, or doesn't it matter?