correct algebra recursive and explicit formulas not accepted

I'm suddenly not able to enter correct answers in the unit on Converting Explicit and Recursive formulas practice and quiz sections of Algebra 1. https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:sequences/x2f8bb11595b61c86:constructing-geometric-sequences/e/sequences-as-functions?modal=1

It worked fine until sometime last week, when I began to get marked wrong even though my answers were right. I tried formatting changes (parentheses, carets, superscript, omitting and adding spaces, etc.) to no avail. I rebooted my Mac, changed to Chrome (KA's recommended browser), rest my keyboard to default, and did a little dance to the gods of online learning. Zilch.
Anyone out there have any ideas? KA's tech support has not been able to be very helpful to date. I'm completely stuck.

Many thanks.