Hey there!
A reciprocal might be an unfamiliar vocab word, but the concept is really quite simple.
It's just a flipped fraction! Multiplying the flipped fraction by it's reciprocal gets you one, and that's how you check your answer.
So we have:
Reciprocal of 9/10 is 10/9, because 9/10 times 10/9 = 90/90 = 1
Reciprocal of 1/11 is 11/1, because 1/11 times 11/1 = 11/11 = 1
Reciprocal of 10(remember, it's 10/1) is 1/10, because 10/1 times 1/10 = 10/10 = 1
Hope this helps!