a: =>15x=210
=>x=14
b: =>2809:x=234
=>x=2809/234
c: =>40x:12=160
=>40x=1920
=>x=48
15.x=2730:13
15.x=210
x=210:15
x=14
_________________________
2808:x=3527-3293
2808:x=234
x=2808:234
x=12
_____________________________
(x.40):12=2400:15
(x.40):12=160
x.40=160.12
x=1920:40
x=48
______________________________
(1260:x)+397=1692-1253
(1260:x)+397=439
1260:x=439-397
x=1260:42
x=30