context: While the silver threepence was minted as a currency coin until nearly the middle of the 20th century, it is clear that the purpose of the coin changed during the reign of George III of the United Kingdom (1760-1820). In the first two years of minting, 1762 and 1763, the coin was obviously produced for general circulation as examples are generally found well worn; on the other hand, coins from the late issue (1817-20) are usually found in very fine condition, indicating that they were probably issued as Maundy money. Over the length of the reign there were several different designs of obverse and reverse in use. Threepences were issued in 1762-63, 1765-66, 1770, 1772, 1780, 1784, 1786, 1792, 1795, 1800, 1817, 1818, and 1820. From 1817 the dimensions of the coin were reduced to a weight of 1.4 grams and diameter of 16 millimetres, following the Great Recoinage of 1816. The inscription on the obverse reads  up to 1800, and  from 1817. The reverse inscription reads  up to 1800 and  from 1817.
question: For how many total years were threepences issued?
answer: 15