a)

\(\left(-11\right).\left(-28\right)+\left(-9\right).13\)

\(=308+\left(-9\right).13\)

\(=308+\left(-117\right)\)

\(=191\)

b)

\(\left(-69\right).\left(-31\right)-\left(-15\right).12\)

\(=\left(-69\right).\left(-31\right)+15.12\)

\(=2139+15.12\)

\(=2139+180\)

\(=2319\)

c)

\(\left[16-\left(-5\right)\right].\left(-7\right)\)

\(=\left[16+5\right].\left(-7\right)\)

\(=21.\left(-7\right)\)

\(=-147\)

d)

\(\left[\left(-4\right).\left(-9\right)-6\right].\left[\left(-12\right)-\left(-7\right)\right]\)

\(=\left[\left(-4\right).\left(-9\right)-6\right].\left[\left(-12\right)+7\right]\)

\(=\left[36-6\right].\left[\left(-12\right)+7\right]\)

\(=30.\left[\left(-12\right)+7\right]\)

\(=30.\left(-5\right)\)

\(=-150\)

Gọi số cần tìm là \(ab\)

Ta có : 

\(\overline{ab}-\overline{ba}=15\)

\(\Rightarrow10a+b-10b-a=45\)

\(\Rightarrow9a-9b=45\)

\(\Rightarrow9\left(a-b\right)=45\)

\(\Rightarrow\left(a-b\right)=45:9\)

\(\Rightarrow a-b=5\)

\(\Rightarrow a=5+b\)

\(\Rightarrow b=0;1;2;3;4\)

\(\Rightarrow a=5;6;7;8;9\)

Vậy \(ab\in\left\{50;94;83;72;61\right\}\)

bn ơi 3,25 có đơn vị đo ko