How many 3-letter code words are possible using the first 10 letters of English alphabet if : no letter can be repeated ?

How many 4-letter code words are possible using the first 10 letters of the English alphabet, if no letter can be repeated ?

How many 3-letter code words are possible using the first 10 letters of English alphabet if : letters are repeated ?

How many 4-letter code can be formed using the first 10 letters of the English alphabet, if no letter can be repeated?

Which letter of English alphabet form simple closed curve?

How many 3-letter words can be made using the letters of the word ‘ORIENTAL’ ?

Serial numbers for an item produced in a factory are to be made using two letters followed by four digits (0 to 9). If the letters are to be taken from the first six letters of English alphabet without repetition and the digits are also not repeated in a serial number, how many serial numbers are possible ?

How many codons are possible with the 4 letter language U, C, A, G?