|Pennant|Name|Hull builder|Ordered|Laid down|Launched|Accepted into service|Commissioned|Est. building cost|Fate|
|---|---|---|---|---|---|---|---|---|---|
|F169|Amazon|Vosper Thornycroft, Woolston|26 March 1969|6 November 1969|26 April 1971|19 July 1974|11 May 1974|£16.8M|To Pakistan as Babur|
|F170|Antelope|Vosper Thornycroft|11 May 1970|23 March 1971|16 March 1972|30 June 1975|16 July 1975|£14.4M|Bombed by Argentine A-4 Skyhawks on 23 May 1982 and sank following day in San Carlos Water|
|F172|Ambuscade|Yarrow Shipbuilders, Scotstoun|11 November 1971|1 September 1971|18 January 1973|23 August 1975|5 September 1975|£16.5M|To Pakistan as Tariq|
|F173|Arrow|YSL|11 November 1971|28 September 1972|5 February 1974|16 May 1975|29 July 1976|£20.2M|To Pakistan as Khaibar|
|F171|Active|Vosper Thornycroft|11 May 1970|21 July 1971|23 November 1972|2 June 1977|17 June 1977|£24.1M|To Pakistan as Shah Jahan|
|F174|Alacrity|YSL|11 November 1971|5 March 1973|18 September 1974|2 April 1977|2 July 1977|£23.8M|To Pakistan as Badr|
|F184|Ardent|YSL|11 November 1971|26 February 1974|9 May 1975|10 September 1977|14 October 1977|£26.3M|Bombed by Argentine A-4 Skyhawks on 21 May 1982 in San Carlos Water and sank following day in Grantham Sound|
|F185|Avenger|YSL|11 November 1971|30 October 1974|20 November 1975|15 April 1978|15 April 1978|£27.7M|To Pakistan as Tippu Sultan|

Question: how many boats costed less than £20m to build?
Answer: 3