diff --git a/study-log.md b/study-log.md
index 1783290..d97206f 100644
--- a/study-log.md
+++ b/study-log.md
@@ -31,6 +31,7 @@ Notes:
 
 Questions:
 - Still unclear on terminology: positive integer, real number, integral number, etc. — these are number classification names, not strictly a set concept. Looking for a reference to solidify these.
+- Reference: https://mathmonks.com/sets/number-sets
 
 Comment:
-- Notes are solid. The number classification question is actually about "number systems" — you'll encounter these more in the lecture notes when they talk about fields and groups. For now, just know that N = natural numbers, Z = integers, R = real numbers. Shortcut: look up "number sets notation" on Khan Academy or Wikipedia. Don't spend too long here — it'll click once you see them used in context.
+- Notes are solid. The number classification question is actually about "number systems" — you'll encounter these more in the lecture notes when they talk about fields and groups. For now, just know that N = natural numbers, Z = integers, R = real numbers. Check out the reference above for a simple breakdown of number sets.