- A600
- B660
- C540
- D720
View written solutionFree
Correct answer: B
- Understand the relation
We have a relation from the set to itself, defined by
Here, can be any element of , while must be an element of that can be written as a product of two primes, each at least .
So the total number of ordered pairs in is:
Since , there are clearly choices for .
Thus we only need to count how many numbers can be written as with prime.
- List primes at least 3 up to 60
The relevant primes are:
We need products .
- Find all possible products
We allow as well.
- With : \quad 3\cdot 5=15, \quad 3\cdot 7=21, \quad 3\cdot 11=33, \quad 3\cdot 13=39, \quad 3\cdot 17=51, \quad 3\cdot 19=57.$$ Next, $3\cdot 23=69>60$, so stop.
- With : \quad 5\cdot 7=35, \quad 5\cdot 11=55.$$ Next, $5\cdot 13=65>60$, so stop.
- With : Next, , so stop.
For larger primes, even the square exceeds , so no more values occur.
Hence the valid values of are:
So the number of valid is
- Count ordered pairs
For each of the valid values of , can be any of the elements.
Therefore,
- Check options
The correct option is which is Option B.
- Compare with stored answer
Stored correct answer: B
Our derived answer: B
So they agree.
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