What does the below rebus riddle means?

Replace the question mark with the correct number in the below-given picture?

Take 9 from 6, 10 from 9, 50 from 40, and leave 6.

How Come ??

John went to meet his friend Jacob, but when he was about to reach the main gate, John notices that Jacob had a mighty dog who was fastened to the tree. The chain is long enough that it allows the dog to reach the main gate.

How can the John reach the main gate?

How many times in a given day, minutes and hour clock comes in a straight line ?

You are given with the following sum. Each of the letters can be decoded as a digit. If we tell you that D = 5, then can you solve it entirely?

DONALD
+GERALD
=ROBERT

What is the next number in this matchsticks series riddle?

I am green and found in the forest and have needles instead of leaves. What am I?
Hint 1: It can be used for Christmas decorations.
Hint 2: I have a strong, scent that is used in air fresheners.

Rearrange the letters in "new door" to make one word.

It is always ahead of me yet I can never see it. What is it?