8 × { 72 : [ 52 - ( 188 - a × 12 ) ] } + 28 = 100
8 × { 72 : [ 52 - ( 188 - a × 12 ) ] } = 100 - 28
8 × { 72 : [ 52 - ( 188 - a × 12 ) ] } = 72
{ 72 : [ 52 - ( 188 - a × 12 ) ] } = 72 : 8
{ 72 : [ 52 - ( 188 - a × 12 ) ] } = 9
[ 52 - ( 188 - a × 12 ) ] = 72 : 9
[ 52 - ( 188 - a × 12 ) ] = 8
188 - a × 12 = 52 - 8
188 - a × 12 = 44
a × 12 = 188 - 44
a × 12 = 144
a = 144 : 12
a = 12