8 x 100 - a = 278
800 - a = 278
a = 800 - 278
a = 522
a - 79 x 10 = 209
a - 790 = 209
a = 790 + 209
a = 999
a + 30 x 10 = 769
a + 300 = 769
a = 769 - 300
a = 469
6 x 9 + a = 300
54 + a = 300
a = 300 - 54
a = 246
a : ( 3 x 3 ) = 10
a : 9 = 10
a = 10 x 9
a = 90
( 6 x 7 ) : a = 6
42 : a = 6
a = 42 : 6
a = 7
