Question: Gerald Grosvenor, 6th Duke of Westminster was succeeded by the British aristocrat who is estimated to be worth how much?

Context:
Gerald Grosvenor, 6th Duke of Westminster
Major General Gerald Cavendish Grosvenor, 6th Duke of Westminster, (22 December 1951 – 9 August 2016) was a British landowner, businessman, philanthropist, Territorial Army general and hereditary peer.
 He was the son of Robert George Grosvenor, 5th Duke of Westminster and Viola Grosvenor.
 He was Chairman of the property company Grosvenor Group.
 He is succeeded by his son, Hugh Grosvenor, 7th Duke of Westminster.
Sally Grosvenor, Duchess of Westminster
Sally Grosvenor, Duchess of Westminster, née Perry (1909 – 30 May 1990), was the wife of Gerald Grosvenor, 4th Duke of Westminster.
La Garganta (finca)
La Garganta (Spanish for "the throat", or "gully") is a private estate, or finca, of around 15,000 ha (32,000 acres) in extent, located in the rugged Sierra Morenain, in rural Ciudad Real province, Castile-La Mancha region, Spain.
 It is situated between the town of Conquista and the hamlet of Minas de Horcajo, in Almodóvar del Campo.
 Around 2003 it was bought on lease and renovated by Gerald Grosvenor, 6th Duke of Westminster.
Gerald Grosvenor, 4th Duke of Westminster
Colonel Gerald Hugh Grosvenor, 4th Duke of Westminster {'1': ", '2': ", '3': ", '4': "} (13 February 1907 – 25 February 1967) was the son of Captain Lord Hugh William Grosvenor and Lady Mabel Crichton and a grandson of Hugh Grosvenor, 1st Duke of Westminster.
Westminster City Council v Duke of Westminster
Westminster City Council v Duke of Westminster was a legal case between Westminster City Council and Gerald Grosvenor, 6th Duke of Westminster heard in November 1990.
 The dispute concerned 532 flats in Page Street, Vincent Street and Regency Street, Pimlico, London.
 These had been designed by architect Sir Edwin Lutyens and erected between 1928-30 for Hugh Grosvenor, 2nd Duke of Westminster.
 In 1937 the Duke assigned the properties - for a peppercorn rent of 1 shilling - to the council on a 999-year lease with the stipulation that they be used only as "dwellings for the working classes... and no other purpose."
Charles Gordon-Lennox, 11th Duke of Richmond
Charles Gordon-Lennox, 11th Duke of Richmond, 11th Duke of Lennox, 11th Duke of Aubigny, 6th Duke of Gordon DL (born 8 January 1955) is a British aristocrat and owner of Goodwood Estate in West Sussex.
 He is the founder of the Goodwood Festival of Speed and the Goodwood Revival.
Nicholas Ashley-Cooper, 12th Earl of Shaftesbury
Nicholas Edmund Anthony Ashley-Cooper, 12th Earl of Shaftesbury Bt, (born 3 June 1979) also known as Nick Ashley-Cooper, is an English peer, landowner and philanthropist.
 He succeeded his brother as Earl of Shaftesbury in 2005.
 The 12th Earl of Shaftesbury is the godson of Gerald Grosvenor, 6th Duke of Westminster, and Simon Elliot, brother-in-law of Charles, Prince of Wales.
Francis Grosvenor, 8th Earl of Wilton
Francis Egerton Grosvenor, 8th Earl of Wilton (born 8 February 1934) is a British aristocrat, financier and academic.
 He is the eldest son of Robert Egerton Grosvenor, 5th Baron Ebury and his first wife Anne Acland-Troyte.
 He succeeded his father as 6th Baron Ebury in 1957, and his fourth cousin, Seymour William Arthur John Egerton, 7th Earl of Wilton, to the earldom in 1999.
Hugh Grosvenor, 7th Duke of Westminster
Hugh Richard Louis Grosvenor, 7th Duke of Westminster (born 29 January 1991), styled as Earl Grosvenor until August 2016, is a British aristocrat, billionaire, businessman and landowner.
 He is the third child and only son of Gerald Grosvenor, 6th Duke of Westminster and his wife Natalia Phillips Grosvenor, Duchess of Westminster.
 He inherited the title of Duke of Westminster on 9 August 2016, on the death of his father.
 The duke is estimated to be worth US$13 billion, making him the world's richest person aged under 30.
Natalia Grosvenor, Duchess of Westminster
Natalia Ayesha Grosvenor, Duchess of Westminster ("née" Phillips; born 8 May 1959) is the widow of Gerald Grosvenor, 6th Duke of Westminster.
 The Duchess will assume the style of Dowager Duchess only upon the marriage of her son.
 At the time of her husband's succession to the title, there were four Duchesses of Westminster, the current Duchess, Sally Grosvenor, Duchess of Westminster, Anne Grosvenor, Duchess of Westminster and Viola Grosvenor, Duchess of Westminster.

Answer: US$13 billion