Example 1
On June 14, 1998, the Chicago Bulls beat the Utah Jazz 87–86 to clinch their sixth NBA title in eight years.
In this sentence, June 14, 1998 is written with numerals since it's a day and a year. 87–86 gets the numeral treatment, too, since it's a score. Finally, it's best to write out eight since it's a one-word number. It's also probably best to pretend Michael Jordan's 2001 "comeback" with the Washington Wizards never happened.
Example 2
Angelo asked his friends, "If I want to give a 20 percent tip on our $54.23 bill for lunch, how much money would that be?"
We didn't know there'd be math on here! This is a grammar lesson! Here, we use the numerals 20 and 54.23 because we're talking about a percentage and an exact amount of money. (And Angelo should leave roughly eleven dollars. Notice that we wrote that one out since it's not an exact amount.)