[ 56 : ( a + 20 ) + 7 ] × 5 - 30 = 15
[ 56 : ( a + 20 ) + 7 ] × 5 = 15 + 30
[ 56 : ( a + 20 ) + 7 ] × 5 = 45
56 : ( a + 20 ) + 7 = 45 : 5
56 : ( a + 20 ) + 7 = 9
56 : ( a + 20 ) = 9 - 7
56 : ( a + 20 ) = 2
a + 20 = 56 : 2
a + 20 = 28
a = 28 - 20
a = 8
