I AM A METAMORPHIC ROCK DARK GREY IN COLOUR I AM USED IN MAKING ROOFS AND BLACKBOARD RIDDLES WITH ANSWERS TO SOLVE - PUZZLES & BRAIN TEASERS

Yes it is possible in Roman Numerals. In Roman Numerals,
9 = IX
10 = X
11 = XI

Thus by removing one from 9, you are getting 10 and by removing one from 11, you are getting 10 again. Did you answer this riddle correctly?
YES  NO  

Luke, because theres no power (its during a blackout). Did you answer this riddle correctly?
YES  NO  

Put two scoops of ice cream on him! Did you answer this riddle correctly?
YES  NO  

It's an udderly bad idea! Did you answer this riddle correctly?
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Cut it in half! Did you answer this riddle correctly?
YES  NO  

Ask a witch to add some sand to it. Did you answer this riddle correctly?
YES  NO  

A candle Did you answer this riddle correctly?
YES  NO  

Racket Did you answer this riddle correctly?
YES  NO  

Santa Did you answer this riddle correctly?
YES  NO  

Piano Did you answer this riddle correctly?
YES  NO  

A school bus. Did you answer this riddle correctly?
YES  NO  

On a tele-bone! Did you answer this riddle correctly?
YES  NO  

Tickle its funny bone! Did you answer this riddle correctly?
YES  NO  

Elves Did you answer this riddle correctly?
YES  NO  

On porpoise! Did you answer this riddle correctly?
YES  NO