(7xa-5+2x6):4+5x10-[(4+6)x5-42]=112
(7xa-5+2x6):4+5x10-[120-42]=112
(7xa-5+2x6):4+5x10-78=112
(7xa-5+2x6):4+50=112+78
(7xa-5+2x6):4+50=190
(7xa-5+2x6):4=190-50
(7xa-5+2x6):4=140
(7xa-5+2x6)=140×4
7xa-5+2x6=560
7xa-5+12=560
7xa-5=560-12
7xa-5=548
7xa=548+5
7xa=553
a=553 : 7
a = 79
