Medium Arithmetic Puzzles

#1544: 3 _ 9 _ 6 _ 6 = -4 medium #1545: 3 _ 9 _ 6 _ 7 = 9 medium #1546: 3 _ 9 _ 6 _ 8 = -6 medium #1554: 4 _ 1 _ 2 _ 8 = 8 medium #1555: 4 _ 1 _ 3 _ 9 = 7 medium #1561: 4 _ 2 _ 3 _ 9 = 10 medium #1562: 4 _ 2 _ 3 _ 9 = -2 medium #1569: 4 _ 2 _ 3 _ 6 = 12 medium #1571: 4 _ 2 _ 3 _ 7 = 13 medium #1575: 4 _ 2 _ 3 _ 9 = -3 medium #1580: 4 _ 2 _ 4 _ 8 = 8 medium #1587: 4 _ 2 _ 4 _ 7 = 14 medium #1593: 4 _ 2 _ 4 _ 6 = 14 medium #1604: 4 _ 2 _ 5 _ 4 = 14 medium #1606: 4 _ 2 _ 5 _ 5 = 15 medium #1607: 4 _ 2 _ 5 _ 6 = 16 medium #1620: 4 _ 2 _ 6 _ 3 = 16 medium #1621: 4 _ 2 _ 6 _ 9 = 12 medium #1625: 4 _ 2 _ 7 _ 2 = 12 medium #1627: 4 _ 2 _ 7 _ 3 = 11 medium #1629: 4 _ 2 _ 7 _ 4 = 10 medium #1645: 4 _ 2 _ 8 _ 5 = -4 medium #1647: 4 _ 2 _ 8 _ 6 = -5 medium #1648: 4 _ 2 _ 8 _ 7 = 8 medium #1649: 4 _ 2 _ 8 _ 7 = -6 medium #1650: 4 _ 2 _ 8 _ 8 = -7 medium #1651: 4 _ 2 _ 8 _ 9 = 10 medium #1654: 4 _ 2 _ 9 _ 3 = 10 medium #1655: 4 _ 2 _ 9 _ 6 = 7 medium #1656: 4 _ 2 _ 9 _ 3 = -2 medium #1666: 4 _ 2 _ 9 _ 8 = 10 medium #1668: 4 _ 3 _ 2 _ 8 = -8 medium #1676: 4 _ 3 _ 2 _ 6 = 12 medium #1677: 4 _ 3 _ 2 _ 7 = 13 medium #1681: 4 _ 3 _ 2 _ 9 = -3 medium #1688: 4 _ 3 _ 4 _ 8 = 10 medium #1697: 4 _ 3 _ 4 _ 6 = 18 medium #1701: 4 _ 3 _ 6 _ 9 = 6 medium #1702: 4 _ 3 _ 6 _ 8 = 8 medium #1703: 4 _ 3 _ 6 _ 3 = 14 medium #1723: 4 _ 3 _ 6 _ 4 = 12 medium #1726: 4 _ 3 _ 6 _ 6 = 14 medium #1731: 4 _ 3 _ 7 _ 7 = 13 medium #1732: 4 _ 3 _ 7 _ 7 = 11 medium #1733: 4 _ 3 _ 8 _ 6 = 8 medium #1734: 4 _ 3 _ 8 _ 2 = -8 medium #1738: 4 _ 3 _ 8 _ 2 = 8 medium #1739: 4 _ 3 _ 8 _ 8 = 11 medium