\( 720 : (90 - x) = 125 - 45 \)
\( 720 : (90 - x) = 80 \)
\( 90 - x = \frac{720}{80} \)
\( 90 - x = 9 \)
\( x = 90 - 9 \)
\( x = 81 \)
Ответ: x = 81.