Well, I think it would be obvious if I had paper and pen, or blocks.
As it was, I did have a visual that came up as you were describing it, because I remember seeing such arrangements before.
And, I look Logical Geometry in 6th grade, was pretty good at it, and have a good memory.
But it wasn’t immediately obvious, and for a problem of that type, I would either want to sketch it or use blocks if I were to be certain my mental image was accurate, and/or to take time to carefully imagine it and reason about it. But I can reason it out in my head without a visual aid, but also having done geometry before and remembering a fair amount of it helps a lot.
For instance, I know and can easily visualize and reason from properties and theorems I remember about equilateral triangles, and from experience with them on paper and in blocks, etc., that the top tip triangle would share a side with an equal triangle, and then if you added two more such triangles, each sharing one of the other two sides of that second triangle, that together they’d be four triangles that make up one larger triangle with sides twice as long as the component triangles. Then you’d just extend it, with the two triangles with sides on one side sharing sides of two new triangles, another triangle between those, and then two triangles at the tips, for a triangle made of nice triangles with side length three times that of the component triangles.