using B tree index with like operator

Question

I have a question about using index in Oracle.
Suppose table has B tree index created on column columnA, which has high cardinality or unique.
This table has 1KK rows.
I make a query example where columnA like 'A%'
Suppose in this example this query will return me 90 percent of data.

So some questions:

1. Will oracle determine to use index or not?
2. If oracle will determine itself, what is the percentage value (example 50%) when oracle will use index. Example 30% use index, 40% will not use? And when I would benefit from using indexes.
3. Can I force oracle to use index or not depending on query myself.

Thanks in advance.