The teacher is up front showing how to do a specific math equation.
A man is demonstrating some math problems on a chalkboard
a man is explaining math problems while doing them on the chalkboard
A man is talking as he does a math problem on the chalk board.
A man is writing math equations on a chalk board.
A person does a complicated math equation on a green board while explaining it.
A guy is explaining a math equation on a chalkboard.
A man writes information about angles on to the chalk board.
A man is writing on a chalkboard explaining the formula regarding the triangle he has drawn.
Using a chalk board, a man teaches how to work a math problem involving triangles.