Andrei:
( a - 70 ) x 3 - 312 = 249
( a - 70 ) x 3 = 249 + 312
( a - 70) x 3 = 561
a - 70 = 561 : 3
a - 70 = 187
a = 187 + 70
Alexandra:
( b + 70 ) : 3 + 312 = 401
( b + 70 ) : 3 = 401 - 312
( b + 70 ) : 3 = 89
b + 70 = 89 x 3
b + 70 = 267
b = 267 - 70