Candidate Key – A Candidate Key can be any column or a combination of columns that can qualify as unique key in database. There can be multiple Candidate Keys in one table. Each Candidate Key can qualify as Primary Key. 

Primary Key – A Primary Key is a column or a combination of columns that uniquely identify a record. Only one Candidate Key can be Primary Key.

• One needs to be very careful in selecting the Primary Key as an incorrect selection can adversely impact the database architect and future normalization. For a Candidate Key to qualify as a Primary Key, it should be Non-NULL and unique in any domain.

Quantity of alohol in the mixture = 40 x 5/8 = 25 lit

Quantity of water = 40 - 25 = 15 lit

According to question,

Required ratio = 20 - 40 x 20100 x 5815 - 40 x 20100 x 38 + 40 x 20100 = 2015 - 3 + 8 = 1 : 1

Let the selling price of the rice = Rs.P/kg

Now, according to the question,

1040 - 100p = 30p

=> p = 8/kg

Hence, the selling price of the rice = Rs. 8/kg

Given 0.875 and 0.375

0.875 ::

After decimal point there are 3 digits 875

Now, mutiply and divide 0.875 with 1000

$$\begin{gathered} 0.875 x 10001000 = 8751000 \\ Now, this can be reduced to fraction 78 \\ Therefore, 0.875 as fraction = 78 \end{gathered}$$

Similarly,

0.375 :: 

After decimal point there are 3 digits 375

Now, mutiply and divide 0.375 with 1000

$$\begin{gathered} 0.375 x 10001000 = 3751000 \\ Now, this can be reduced to fraction 38 \\ Therefore, 0.375 as fraction = 38 \end{gathered}$$

The Riddle itself has the Answers.

What is a 4 letter word

Yet is written in 3 letters

Everything is written with 9 letters

Then with 4 letters

Rarely has 6 letters

Never is written in 5 letters.

One grandfather, One father and One son makes two fathers and two sons.

3/8 as a Decimal :

In this we should convert the denominator into 10, 100, 1000, 10000, 100000,... to make it simple.

Now to make 8 into 1000, we multiply numerator and denominator with 125

=> 3 x 1258 x 125 = 3751000 = 0.375

1/10 as a Decimal :

1 divided into 10 parts of each 0.1.

Therefore, 3/8 = 0.375 and 1/10 = 0.1 in decimals.

Let the total amount be 200 {L.C.M of 40 and 50}

Chacobar C.P. = 200/50 = 4

Fivestar C.P = 200/40 = 5

Remaining Money after petrol = [200 - 200×10%] = 180

Remaining money after buying fivestars = [180 - 20×5] = 80

So number of Chacobar she can buy = 80/4 = 20