Question

Write three pairs of co-primes between $15$ and $30.$

• A. $19$ and $23,23$ and $25,$ & $27$ and $29$
• B. $16$ and $21,18$ and $25,$ & $20$ and $27$
  
• C. $16$ and $22,18$ and $26,$ & $20$ and $27$
  
• D. none of these

Answer 1

Answer: (B)

$16$ and $21,18$ and $25,$ & $20$ and $27$

Two integers $a$ and $b$ are said to be relatively prime, mutually prime, or co-prime if the only positive integer that evenly divides both of them is $1.$ That is, the only common positive factor of the two numbers is $1.$
They are $16$ and $25,27$ and $20,21$ and $22.$