a: =>13*ab=900+ab
=>12ab=900
=>ab=75
b: =>(800+ab)/ab=17
=>800=16ab
=>ab=50
c: =>10abc+5-abc=1112
=>9abc=1107
=>abc=123
d: =>(100+ab)*126=201*100+ab
=>125ab=7500
=>ab=60
e: =>100ab+ab+ab=1326
=>102ab=1326
=>ab=13
f: =>30000+abc=241abc
=>30000=240abc
=>abc=125