Solution:

Once upon a time

Solution:

80

Explanation:
21 B 7 C 9 D 10 A 13
= 21 / 7 + 9 X 10 - 13
= 3 + 90 - 13
= 80

Solution:

Mixing business with pleasure.(PbLuEsAiSnUeRsEs)

Solution:

Muz is the required code word.

In the statements (1), (2) and (3), common coded word is 'Stew' and common normal word is 'are'. Therefore, 'Stew' = 'are'.
k
In the statements (1) and (4), common coded word (except 'are') is 'Choi' and common normal word is 'Spirits'. Therefore, 'Choi' = 'Spirits'.

Likewise, in the statements (2), (3) and (4), common coded word is 'Fru' and common normal word is 'Sinister'. So, 'Fru' = 'Sinister'.

Hence, 'Choi', 'Stew', 'Fru' means 'Spirits', 'are', 'Sinister' respectively. Thus, in statement (4), the remaining word i.e., 'Muz' = 'the'.

Solution:

QHJRPBWF

CYLINDER = UHGQGJWA
Reverse the letters of CYLINDER
REDNILYC
Position increase in first four letters by 3.
REDN becomes UHGQ

Position decrease in last four letters by 2.
ILYC becomes GJWA.

Thus in the same manner
HYDROGEN = QHJRPBWF

Solution:

1. 8P of the SS - 8 planets of the Solar System
2. 1M of the E - 1 Moon of the earth
3. 1S of the SS - 1 Sun of the Solar System
4. 60M in 1H - 60 minutes in 1 Hour

Solution:

1. 26 L of the A (26 Letters of the Alphabet)

2. 7 D of the W (7 days of the Week)

3. 1000 Y in a M (1000 years in a millennium)

Solution:

Wise you are, wise you be, I see you are too wise for Me

Solution:

What goes up must come down.

Solution:

Hard, using the first letter of the first word, the second letter of the second word, etc