300 - 5 400 : (24 + х) = 210
5 400 : (24 +х) = 300 - 210
5 400: (24 +х) = 90
24 +х =5 400 : 90
24 +х =60
х = 60 - 24
х = 36
проверка :
300 - 5 400 : (24 + 36 ) = 210
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