The son is 3 units and the daughter is 1 unit. Both totals share Maria, so subtracting them leaves son − daughter = 2 units.

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

1. (Maria + son) − (Maria + daughter) → 100 − 70 = 30
2. Son − daughter = 2 units → 30
3. 1 unit (daughter) → 30 ÷ 2 = 15; son = 45
4. Maria → 70 − 15 = 55 kg

Maria weighs 55 kg.

Check: Maria 55 + son 45 = 100; Maria 55 + daughter 15 = 70; 45 = 3 × 15. ✓