The animation showing all of the triangles, one by one, is available above when you are signed in to your Transum subscription account. If you do not yet have an account and you are a teacher or parent you can apply for one here.
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Topics: Starter  Combinations  Problem Solving  Shape
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Previous Day  This starter is for 23 May  Next Day
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There is a printable worksheet to go with this activity.
Other shape counting starters:
How Many Squares 1?
 How Many Squares 2?
How Many Triangles 1?

How Many Triangles 2?  How Many Triangles 3?
How Many Rectangles?
 Rectangles Investigation 
Icosahedron 
Mystic Rose
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Teacher, do your students have access to computers such as tablets, iPads or Laptops? This page was really designed for projection on a whiteboard but if you really want the students to have access to it here is a concise URL for a version of this page without the comments: Transum.org/go/?Start=May23 However it would be better to assign one of the student interactive activities below. 

Here is the URL which will take them to a related student activity.
See the National Curriculum page for links to related online activities and resources.