Răspuns :
x - al treilea nr
( 10 + 12 + x ) : 3 = 13
22 + x = 13 x 3
22 + x = 39
x = 39 - 22
x = 17
( 10 + 12 + x ) : 3 = 13
22 + x = 13 x 3
22 + x = 39
x = 39 - 22
x = 17