Yennie is one unit (= Peter) plus an extra 5 kg. Together they are 103 kg.

Plain numbers are given; italic underlined numbers are worked out.

1. 2 units + 5 → 103 kg
2. 2 units → 103 − 5 = 98 kg
3. 1 unit (Peter) → 98 ÷ 2 = 49 kg
4. Yennie → 49 + 5 = 54 kg

Yennie is 54 kg.

Check: Peter 49 + Yennie 54 = 103 kg, and 54 − 49 = 5 kg. ✓