(2 + x) + (4 + x) + (6 + x) + ... + (52 + x) = 780
26x + (2 + 4 + 6 + ... + 52) = 780
26x + 702 = 780
26x = 780
x = 780 : 26
x = 30
(2 + x) + (4 + x) + (6 + x) + ... + (52 + x) = 780
26x + (2 + 4 + 6 + ... + 52) = 780
26x + 702 = 780
26x = 780
x = 780 : 26
x = 30