A teacher explains chemistry equations on a chalk board at a school.
A teacher is at a black board explaining a chemistry problem.
A man is explaining a scientific equation on a green board.
a professor is talking and pointing at information on the chalk board
A man was teaching the chemistry equation by writing on the board and explains that
A man is standing at a blackboard and giving a chemistry lecture.
A man points and gestures to several mathematical equations on a chalkboard.
A foreign language chemistry teacher is demonstrating a chemical reaction on the blackboard.
A man speaks in front of a chalkboard about equations which are drawn on it.
An instructor is explaining a complex formula written on a chalkboard.