Which of the following numbers are co-prime?
(a)18 and 35 
(b)15 and 37 
 (c) 30 and 415 
(d) 17 and 68
(e)216 and 215     
(f) 81 and 16

Explanation

To determine which of the given pairs of numbers are co-prime, we need to check if they have any common factors other than 1. Co-prime numbers are defined as numbers that have no common factors except for 1.

Let's analyze each pair step by step:

### Step 1: Check 18 and 35
- **Factors of 18**: 1, 2, 3, 6, 9, 18
- **Factors of 35**: 1, 5, 7, 35
- **Common Factors**: The only common factor is 1.
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