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) 16 and 215     
(f) 81 and 16

Explanation

To determine which of the given pairs of numbers are co-prime, we need to find the factors of each number in the pairs and check if they have any common factors other than 1. If they do not share any common factors other than 1, they are co-prime.

Let's analyze each pair step by step:

### Step 1: Check Pair (a) 18 and 35
- **Factors of 18**:
- 18 = 2 × 3 × 3 = 2 × 3²
- Factors: 1, 2, 3, 6, 9, 18
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