Maths-
General
Easy
Question
If r, s, t are prime numbers and p, q are the positive integers such that the LCM of p, q is r2t4s2, then the number of ordered pair (p, q) is –
- 224
- 225
- 252
- 256
Hint:
Natural numbers known as prime numbers can only be divided by one (1) and by the number itself. In other terms, prime numbers are positive integers greater than one that only have the number itself and the number's first digit as factors. Here we have given r, s, t are prime numbers and p, q are the positive integers such that the LCM of p, q is r2t4s2, then what is the number of ordered pair (p, q).
The correct answer is: 225
An ordered pair is made up of the ordinate and the abscissa of the x coordinate, with two values given in parenthesis in a certain sequence.
Now we have given the LCM as: r2t4s2
Consider following cases:
Case 1: if p contains r2 then q will have rk, for the value k=0,1.
So 2 ways.
Case 2: if q contains r2 then p will have rk, for the value k=0,1.
So 2 ways.
Case 3:Both p and q contain r2
So 1way.
So after this we can say that:
exponent of r=2+2+1 = 5 ways.
Similarly
exponent of t=4+4+1=9 ways.
exponent of s=2+2+1 = 5 ways.
So total ways will be:
5 x 5 x 9 = 225 ways.
Finding the smallest common multiple between any two or more numbers is done using the least common multiple (LCM) approach. A number that is a multiple of two or more other numbers is said to be a common multiple. Here we understood the concept of LCM and the pairs, so the total pairs can be 225.
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