In 2 years' time, Mary : Sister = 7 : 10.

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• Sister now = 18 (given answer). Sister in 2 years = 20
• Mary in 2 years = \(\frac{7}{10}\) × 20 = 14
• Mary now = 14 − 2 = 12
• Check ratio now: 12 : 18 = 2 : 3
• The age gap = 18 − 12 = 6 years (never changes)

Mary's sister is 18 years old now.
Check: In 2 years, 14 : 20 = 7 : 10. ✓