diff --git a/core/numeric/convert.xml b/core/numeric/convert.xml
index 27174fa0..c5a056a2 100644
--- a/core/numeric/convert.xml
+++ b/core/numeric/convert.xml
@@ -413,4 +413,3 @@
-
diff --git a/core/numeric/percent.xml b/core/numeric/percent.xml
new file mode 100644
index 00000000..499c747c
--- /dev/null
+++ b/core/numeric/percent.xml
@@ -0,0 +1,94 @@
+
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+
+ Converting between real and~percent is trivial:
+
+ $$
+ \begin{align}
+ r &= p\over100;
+ p &= 100r.
+ \end{align}
+ $$
+
+ ^[Percent difference] (also called ^[relative change]) describes
+ the~change in value as a~percentage (for example: $10$~is $50%$
+ of~$20$, and is~$100%$ of~$5$). ^[Percent change] recognizes the
+ direction of the change (that is---$10$ is~$20$ reduced by~$50%$, so
+ $p=-50$).
+
+ Since ``change'' sounds like a~verb eclarative contexts, we use
+ the~term ^[percent difference] to mean ^[relative change]; it is
+ defined as:
+
+ $$\delta \over x = {{x_\beta - x_\alpha}\over{x_\alpha}} \times 100 = p.$$
+
+ Notice that this preserves the direction of the change. So, in the
+ context of the previous example, we can say that $10$
+ \emph{reduces}~$20$ by~$50%$, giving~$p=-50%$.
+
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+ Real number to convert into percentage
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+ false
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+ false
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diff --git a/core/test/core/numeric/percent.xml b/core/test/core/numeric/percent.xml
new file mode 100644
index 00000000..36169fc1
--- /dev/null
+++ b/core/test/core/numeric/percent.xml
@@ -0,0 +1,142 @@
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+ These tests are similar to the above, except that they represent
+ the~percent difference between two values. That is, given the
+ values $5$~and $15$, we arrive at a~$200%$ increase. So
+ if~$p=200$, then we want $5r=15, r=3=\((15-5)/5\)+1$.
+
+
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+
+ Similarly, if we go from~$10$ to~$5$, we have~$p=-100$
+ and~$10r=5$; then~$r=1/2=0.5$.
+
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