diff --git a/datetimes.Rmd b/datetimes.Rmd
index a1a3e0b..42c9600 100644
--- a/datetimes.Rmd
+++ b/datetimes.Rmd
@@ -310,7 +310,7 @@ Setting larger components of a date to a constant is a powerful technique that a
 1.  Compare `dep_time`, `sched_dep_time` and `dep_delay`. Are they consistent?
     Explain your findings.
 
-1.  Compare `airtime` with the duration between the departure and arrival.
+1.  Compare `air_time` with the duration between the departure and arrival.
     Explain your findings. (Hint: consider the location of the airport.)
     
 1.  How does the average delay time change over the course of a day?
diff --git a/transform.Rmd b/transform.Rmd
index 7d98099..06dd2d3 100644
--- a/transform.Rmd
+++ b/transform.Rmd
@@ -356,7 +356,7 @@ There are many functions for creating new variables that you can use with `mutat
 *   Arithmetic operators: `+`, `-`, `*`, `/`, `^`. These are all vectorised,
     using the so called "recycling rules". If one parameter is shorter than 
     the other, it will be automatically extended to be the same length. This 
-    is most useful when one of the arguments is a single number: `airtime / 60`,
+    is most useful when one of the arguments is a single number: `air_time / 60`,
     `hours * 60 + minute`, etc.
     
     Arithmetic operators are also useful in conjunction with the aggregate
@@ -453,7 +453,7 @@ ggplot(flights, aes(air_time - airtime2)) + geom_histogram()
     Convert them to a more convenient representation of number of minutes
     since midnight.
     
-1.  Compare `airtime` with `arr_time - dep_time`. What do you expect to see?
+1.  Compare `air_time` with `arr_time - dep_time`. What do you expect to see?
     What do you see? What do you need to do to fix it?
     
 1.  Compare `dep_time`, `sched_dep_time`, and `dep_delay`. How would you