He flipped the coin 39 times, of which 26 heads came up. That must mean 39-26 = 13 tails came up.
The ratio of heads to tails is 26/13 = 2/1
We can write that as 2:1 to mean similar notation. The key is that we list heads first since the phrasing is "ratio of heads to tails", where heads is listed first. If you had tails first, then it would flip to 1/2 aka 1:2
