Let the cost price be x. Selling Price = x - (15/100 * x) = 255. Thus, 0.85x = 255, x = 300.

Selling Price = $400 - (25% of $400) = $400 - $100 = $300. Profit = Selling Price - Cost Price = $300 - $250 = $50.

Let the cost price be $100. Marked Price = $100 + ($100 * 0.50) = $150. Selling Price after discount = $150 - ($150 * 0.20) = $120. Effective increase = (($120 - $100) / $100) * 100 = 20%.

Substituting n = 7 into the formula gives a_7 = 3(7) + 1 = 21 + 1 = 22.

Substituting n = 5 into the equation gives a_5 = 3(5) + 2 = 15 + 2 = 17.

a_5 = 2(5) + 3 = 13.

Substituting n = 7 into the formula gives a_7 = 3(7) + 2 = 21 + 2 = 23.

The resulting set will be {2, 4} as these are the only even numbers in set A.

The number 10 does not satisfy the constraint x < 10, hence it is not a member of set A.

Set D includes all positive integers from 1 to 15, which totals 15 elements.

The number 4 is not a prime number, hence it is not included in the set of prime numbers less than 20.

The fruit 'Banana' would not be included in the set as it is not red.

The letter E is a vowel and therefore not included in the set of consonants.

The letter A is an element of the set as it is a vowel in the English alphabet.

The letter Z is not a vowel and therefore is not an element of the set defined by vowels.

The vowels in the English alphabet are A, E, I, O, and U. The letter Y is sometimes considered a vowel but is not classified as one in this context.

The valid element of the set is 5, as it is a natural number and less than 10.

The correct representation of the set is {1, 2, 3, 4, 5, 6, 7, 8, 9}, as it includes all natural numbers less than 10.

The largest natural number less than 10 is 9, making it the largest element in the defined set.

The element '5' is not part of the set because the constraint specifies that x must be less than 5.

The even natural numbers less than 20 are 2, 4, 6, 8, 10, 12, 14, 16, and 18. Therefore, 10 is included in the set.

The divisors of 12 are 1, 2, 3, 4, and 6, so 5 is not an element of this set.

The number '10' is not a multiple of 3, hence it is not an element of the set.

The set includes prime numbers less than 10, which are 2, 3, 5, and 7. The number 4 is not a prime number.

The number 4 is not a prime number, hence it is not included in the set of prime numbers less than 20.

The number 4 is not a prime number, hence it is not included in the set of prime numbers less than 20.

The number 21 is not included in the set as it does not meet the criteria of being less than 20.

All the operations listed can be validly performed on the set defined by the constraints.

1 is a natural number, while -1 and 0 are not considered natural numbers, and 2.5 is not an integer.

5 is a natural number and is less than 10, making it a valid element of the set defined by the given constraints.