Hi Amity007,Great question, and it's the exact spot where almost everyone in this thread got stuck too. The key is what the variable
S actually stands for.
S is the percent change, not the size of the decrease. The problem defines it with a signed formula: (X_New − X_Old) / X_Old ×
100. Because enrollment fell, that formula already spits out a
negative number for S. So the minus sign you're looking for isn't missing - it's baked
inside S.
That's why the students-in-2004 factor is written as
(100 + S)/100, not (100 − S)/100:
- "
100" means the original
100% of students.
- Adding S adjusts for the change.
- Since S is already negative,
adding it
lowers the total - exactly the decrease you expect.
If you instead wrote (100 − S), you'd be subtracting a negative, which
flips it back to an increase - the opposite of what happened.
See it with real numbersSay enrollment dropped
20%. Then by the formula, S =
-20.
- Correct form:
100 + S =
100 + (
-20) =
80 →
80% of the original. ✓ (a
20% drop)
- Wrong form:
100 − S =
100 − (
-20) =
120 →
120%. ✗ (a
20%rise)
So (100 + S) faithfully captures the decline; (100 − S) would undo it.
This is why the answer uses
(100+S)/(100+F) × R rather than the option with (100−S). The algebra holds no matter the sign - you just plug in the signed value of S, and the decrease takes care of itself. Same logic applies to F: faculty grew,
therefore F is positive, and
100 + F correctly increases that count.
Answer: Column 1 = 5,500/R; Column 2 = (100+S)/(100+F) × RAmity007
Why aren't we taking -ve sign into consideration even when it is already mentioned that students enrolments have decreased from fall 1999 to spring 2004?