Răspuns :
Am pus o poza cu rezolvarea
77-(7+n:7)x7=7 <=>
77-7x7-n:7x7=7 <=>
77-49-n=7 <=>
77-49-7 = n <=>
21=n
n=21
77-(7+n:7)x7=7 <=>
77-7x7-n:7x7=7 <=>
77-49-n=7 <=>
77-49-7 = n <=>
21=n
n=21
77 - (7+n:7)*7=7
(7+n:7)*7= 77-7
(7+n:7)*7=70
7+n:7= 70:7
7+n:7=10
n:7=10-7
n:7=3
n=3x7
n=21
(7+n:7)*7= 77-7
(7+n:7)*7=70
7+n:7= 70:7
7+n:7=10
n:7=10-7
n:7=3
n=3x7
n=21