10 × { 24 - [ 20 - ( 5 × a - 3 ) ] : 2 } = 150
{ 24 - [ 20 - ( 5 × a - 3 ) ] : 2 } = 150 : 10
{ 24 - [ 20 - ( 5 × a - 3 ) ] : 2 } = 15
{ [ 20 - ( 5 × a - 3 ) ] : 2 } = 24 - 15
{ [ 20 - ( 5 × a - 3 ) ] : 2 } = 9
[ ( 5 × a - 3 ) : 2 ] = 20 - 9
[ ( 5 × a - 3 ) : 2 ] = 11
5 × a - 3 = 11 × 2
5 × a - 3 = 22
5 × a = 22 + 3
5 × a = 25
a = 25 : 5
a = 5
