72-[21-(75-56×b÷a-23)÷4+3]+7=61
72-[21-(75-56×b÷a-23)÷4+3] = 61 - 7
72-[21-(75-56×b÷a-23)÷4+3] = 54
[21-(75-56×b÷a-23)÷4+3] = 72-54
21-(75-56×b÷a-23)÷4+3 = 18
21-(75-56×b÷a-23)÷4 = 18-3
21-(75-56×b÷a-23)÷4 = 15
(75-56×b÷a-23)÷4 = 21-15
(75-56×b÷a-23)÷4 = 6
75-56×b÷a-23 = 6×4
75-56×b÷a-23 = 24
75-56×b÷a = 24+23
75-56×b:a = 47
56×b:a = 75-47
56×b:a = 28
b:a = 28:56
b:a = 1/2
b/a = 1/2
a = 2b
am facut verificarea si solutia este adevarata.
