minor edits to 'learn to code'

Author: alexch

sites/en/learn-to-code/extra.md

@@ -13,4 +13,3 @@ We've gone over the basics, but Ruby has lots of other language features. Here's
 [Methods](http://codelikethis.com/lessons/ruby_objects/objects#behavior) and [Classes](http://codelikethis.com/lessons/ruby_objects/classes) are the heart of [Object-Oriented Programming](https://en.wikipedia.org/wiki/Object-oriented_programming). They
 
 ## ...and [lots more](http://codelikethis.com/lessons)...
-

sites/en/learn-to-code/numbers.md

@@ -28,8 +28,8 @@ Q: What is 1 plus 2 times 3?
 
 A: *It depends!*
 
-    (1 + 2) * 3 == 9
-    1 + (2 * 3) == 7
+  * `(1 + 2) * 3` is 9
+  * `1 + (2 * 3)` is 7
 
 # Parentheses Are Free
 
@@ -62,9 +62,9 @@ Don't panic! The solution is easy.
 Numbers know a message that converts them into strings. `to_s` means "to string".
 
     "1" + 2.to_s
-    
+
 Likewise, strings know a message that converts them into numbers.
-    
+
     1 + "2".to_i
 
 `to_i` means "to integer".
@@ -75,7 +75,7 @@ Try this in irb!
 
 # WTFixnum?
 
-The error said `can't convert Fixnum into String`. 
+The error said `can't convert Fixnum into String`.
 
 Q: What is a Fixnum?
 

sites/en/learn-to-code/strings.md

@@ -41,16 +41,3 @@ Definitely try these out in irb! It's pretty fun.
 * What does your name look like, repeated 1000 times?
 * What is the tenth character of "Matz is nice"? (Trick question!)
 
-# Interpolation
-
-    first = "Joe"
-    last = "Smith"
-    
-`+` does *concatenation*
-    
-    full = first + " " + last
-
-`#{}` does *interpolation*
-
-    full = "#{first} #{last}"
-

sites/en/ruby/strings.step

@@ -35,6 +35,21 @@ IRB
   message 'The code in the curly braces can be any valid Ruby statement. Try putting various things in the curly brackets to see what works and what doesn\'t.'
 end
 
+tip <<-'MD'
+# Concatenation vs. Interpolation
+
+    first = "Joe"
+    last = "Smith"
+
+`+` does *concatenation*
+
+    full = first + " " + last
+
+`#{}` does *interpolation*
+
+    full = "#{first} #{last}"
+MD
+
 step do
   message "Try out some of these String methods."
   irb <<-IRB