Find The Next Number In The Sequence

30 10 15 13 0 16 -15 ?

Replace the question mark with the correct number below.

What is always in front of you but can’t be seen?

A horse was tied to a rope 5 meters long and the horses food was 15 meters away from the horse. How did the horse reach the food?

John: "Hi Teacher, What is your favourite movie ?".
Chemistry Teacher: "Indium Cerium Platinum Iodine Oxygen Nitrogen".
Ten seconds later... John replied I got it.

Do you?

A worker is to perform work for you for seven straight days. In return for his work, you will pay him 1/7th of a bar of gold per day. The worker requires a daily payment of 1/7th of the bar of gold. What and where are the fewest number of cuts to the bar of gold that will allow you to pay him 1/7th each day?

Without looking at a calendar, within a minute name, a boy's name using 5 consecutive 1st letters of 5 consecutive months

What does this rebus means ?

Through your logical capabilities, can you make 7 an even?

What number does the below three word/phrase identify and why?
1. Deck
2. Year
3. Singing in the Rain