x*5439=54*5438+54

X x 5439 = 54 x ( 5438 + 1 )

X x 5439 = 54 x 5439

X = 54

7899*x=102*7890+102+192*8

X x 7899 = 102 x ( 7890 + 1 ) + 1536

X x 7899 = 806418

X = 102 , 0911508

$\begin{array}{l} x \times 5439 = 54 \times 5438 + 54{\rm{ }}\\ {\rm{x}} \times 5439 = 293652 + 54\\ x \times 5439 = 293706\\ x = 293706:5439\\ x = 54\\ Vay:x = 54\\ 7899 \times x = 102 \times 7890 + 102 + 192 \times 8\\ 7899 \times x = 804780 + 102 + 1536\\ 7899 \times x = 804882 + 1536\\ 7899 \times x = 806418\\ x = 806418:7899\\ x = 102,09\\ vay:x = 102,09 \end{array}$