Brain Teasers and Puzzles 

Workings

Hint
You might need an old calculator.

Answer
Coil.

Reasoning
This is the only word that cannot be spelled using a calculator.

For example, if you enter 376616 into an old-fashioned calculator, and look at the number upside down, you can read GIGGLE.
376616  = giggle
7105    = soil
57738   = bells
378806  = gobble
4506    = gosh
5508    = boss
3704    = hole
5355378 = blesses
coil    = ????

Workings

Hint
One of the words begins with the letter S.

Answers

Workings

Hint
Both words begin with the letter B.

Answer
Brainy and binary.

Workings

Hint
Think of the alphabet.

Answer
25 for Y.

These are the positions of the letters of the alphabet, and they are spelling Donkey.