( 87 - х * 9 ) * 40 =960
87 -х * 9 = 960 : 40
87 - х * 9 = 24
х * 9 = 87 - 24
х * 9 =63
х = 63 : 9
х =7
проверка :
( 87 - 7 * 9 ) * 40 = 960
960 = 960
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