How many students dropped out?
4/198 = about 0.0202 of the students dropped out.
But we want percent, so it's about 2.02% of the students dropped out.
.
How many students didn't drop out?
(198-4)/198 = 194/198 = about 0.9797 of the students dropped out.
But we want percent, so it's about 97.97% of the students dropped out.
.
Extras:
If 2.02% of the students dropped out and 97.97% of the students stayed, what happened to the other 0.01% of them? 2.02% + 97.97% = 99.99%, so don't we have 0.01% missing?
The part above where it says "about 0.0202" would be more accurate if we said "0.02020202 (02's repeating forever)." The part where it says "about 0.9797" would be more accurate as "0.97979797 (97's repeating forever)."
0.02020202 (02's repeating forever) + 0.97979797 (97's repeating forever) = 0.99999999 (9's repeating forever), which is very very close to 1, aka 100%.