Another oldie (corrected)

[soroban]

We can arrange nine coins and make eight rows of three coins each.

      o - o - o
      | \ | / |
      o - o - o
      | / | \ |
      o - o - o

Now arrange the nine coins and make ten rows of three coins each.

 

[soroban]

I called it an "oldie" because I saw this one many many years ago.
And I was sure that someone had seen it . . . or that someone could solve it.

The solution is disarmingly simple . . .

*In the middle row, note that coins on the ends.
Move both of them a half-unit inward.
Now count the rows-of-3.*