a$.$ The largest$$ single$-$day point loss of a$$ country's major stock market occurred when the market lost $2893$ points and closed at $20$ comma $263.$ What was the percentage$$ change?

b$.$ The largest$$ single$-$day percentage loss of a$$ country's major stock market occurred when the market closed down $25\mathrm{.}1\%$ at $1854.$ What was the point$$ change?