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
, then the number of ordered pair (p, q) is-
- 224
Â
Â
- 225
Â
Â
- 252S
Â
Â
- 256
Â
Â
Hint:
At least one of the p and q, must have r,s and t in their prime factorization.
Now, we have to consider each case for selecting the number of ways for power of r, s and t . So, in this way we have to approach the problems.
The correct answer is: 225
Step by step solution :
It is given that 
That is, at least one of p and q must haveÂ
,Â
 andÂ
 in their prime factorizations.
Now, consider the cases for power of r as follows:
Case 1 : p contains Â
 then q has Â
 with K = (0,1,2,3)
That is, number of ways=4.
Case 2 : q contains Â
 then q has Â
 with K = (0,1,2,3)
That is, number of ways=4.
Case 3 : Both p and q containsÂ
Â
Then, number of ways=1.
Therefore, exponent of r may be chosen in 2+2+1=5 ways.
Similarly, exponent of s may be chosen in 4+4+1=9 ways andÂ
Exponent of s may be chosen in 2+2+1=5 ways
Thus, the total number of ways is:
 
Hence, the number of the ordered pair (p, q) is 225.
Be careful about selecting the r s and t for this particular problem. Because number ways for each selection of r, s and t there are 3 cases. Because there are two values given
p, q . Hence to calculate the total number of ways is to multiply the number of ways for r, number of ways for s and number of ways for t. So, in this way to solve this particular type of problem.
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