[15×(10a-11)-120]×10-125=25
[15×(10a-11)-120]×10 = 25 + 125
[15×(10a-11)-120]×10 = 150
15×(10a-11)-120 = 150 : 10
15 × (10a-11)-120 = 15
15 × (10a-11) =15 + 120
15 × (10a-11) = 135
10a - 11 = 135 : 15
10a - 11 = 9
10a = 9 + 11
10a =20
a = 20 : 10
a = 2