Video Explanation

Text Explanation

Let's write some equations based on the information we've been given. We know that k is twice its reciprocal. We can write this as: 

k = 2 

We can simplify the right hand side, and then multiply both sides of the equation by k: 

k = 

 = 2

This means that k is equal to either  or . However, the first sentence in the question tells us that k is a positive number. That means that k cannot be equal to , so it must be the case that 

k = 

The question also tells us that j is twice k. So: 

j = 2k

We know that k = , so we can write: 

j = 2 

We now have values for j and k, so we can find the value of jk: 

j  k = (2 ) 

j  k = 4

So jk = 4, and the answer is B. 

FAQ: Is there another way to solve this?

Actually, yes. And in this case, the other way is just as quick as the method in the explanation video and the text explanation above :-)

The question tells us that k = 2 , which means we have this:
k = 

It also tells us this:
j = 2k

So in order to find jk, we can just substitute the values:
j  k 

= 2k  2/k 

=
= 4

This might not be the most intuitive approach for everybody, though--you have to see ahead of time that the ks will cancel out--which is why that's not the method in the video. But if you used it, then great! It definitely gets to the answer quickly. And that's what counts!