计算：
(1)$(-4)× (+7)$； (2)$(-3)× (-9)$； (3)$\left(-99\frac{3}{5}\right)× 0$；
(4)$(-9)× \left(-\frac{2}{3}\right)$； (5)$(-0.45)× \left(-\frac{4}{9}\right)$； (6)$\left(-2\frac{2}{5}\right)× \left(+\frac{3}{4}\right)$；
(7)$-7× 21$； (8)$(-10.2)× \left(-2\frac{1}{2}\right)$； (9)$(+5.6)× (-1.2)$；
(10)$(-3.48)× (-0.7)$； (11)$1\frac{3}{4}× (-8)$； (12)$(-4)× 1.25$；
(13)$\left(-1\frac{2}{3}\right)× \left(-\frac{1}{5}\right)$； (14)$(-0.7)× \left(-\frac{15}{4}\right)$； (15)$\left(+1\frac{2}{3}\right)× \left(-2\frac{2}{5}\right)$.
答案：(1)-28 (2)27 (3)0 (4)6 (5)0.2 (6)$-\frac{9}{5}$ (7)-147 (8)$\frac{51}{2}$ (9)-6.72 (10)2.436 (11)-14 (12)-5 (13)$\frac{1}{3}$ (14)$\frac{21}{8}$ (15)-4
解析：
(1)解：$(-4)×(+7)=-4×7=-28$
(2)解：$(-3)×(-9)=3×9=27$
(3)解：$\left(-99\frac{3}{5}\right)×0=0$
(4)解：$(-9)×\left(-\frac{2}{3}\right)=9×\frac{2}{3}=6$
(5)解：$(-0.45)×\left(-\frac{4}{9}\right)=0.45×\frac{4}{9}=0.05×4=0.2$
(6)解：$\left(-2\frac{2}{5}\right)×\left(+\frac{3}{4}\right)=-\frac{12}{5}×\frac{3}{4}=-\frac{3}{5}×3=-\frac{9}{5}$
(7)解：$-7×21=-147$
(8)解：$(-10.2)×\left(-2\frac{1}{2}\right)=10.2×\frac{5}{2}=5.1×5=25.5=\frac{51}{2}$
(9)解：$(+5.6)×(-1.2)=-5.6×1.2=-6.72$
(10)解：$(-3.48)×(-0.7)=3.48×0.7=2.436$
(11)解：$1\frac{3}{4}×(-8)=\frac{7}{4}×(-8)=7×(-2)=-14$
(12)解：$(-4)×1.25=-5$
(13)解：$\left(-1\frac{2}{3}\right)×\left(-\frac{1}{5}\right)=\frac{5}{3}×\frac{1}{5}=\frac{1}{3}$
(14)解：$(-0.7)×\left(-\frac{15}{4}\right)=\frac{7}{10}×\frac{15}{4}=\frac{7}{2}×\frac{3}{4}=\frac{21}{8}$
(15)解：$\left(+1\frac{2}{3}\right)×\left(-2\frac{2}{5}\right)=\frac{5}{3}×\left(-\frac{12}{5}\right)=-4$
