\(a,72,9\times99+72+0,9\)
\(=0,9+72\times99+72+0,9\)
\(=72\times\left(99+1\right)+0,9\times\left(1+1\right)\)
\(=72\times100+0,9\times2\)
\(=7200+1,8=7201,8\)
\(b,2,5\times0,2\times4\times5\)
\(=\dfrac{5}{2}\times\dfrac{1}{5}\times20\)
\(=\dfrac{1}{2}\times20=10\)

4 và 5