Yes, she is correct. There are nine edges on a triangular prism and, when you multiply a whole number by nine, the answer is always a multiple of nine.
Try this
Grace is measuring the edges of different sized triangular prisms.
The length of each edge measures a whole number of centimetres.
She thinks that the sum of the edges on each triangular prism will be a multiple of nine.
Is she correct? Explain how you know.
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