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食品 refers to all the eatable stuff. It generally means food.
饭 originally refers to rice, and 菜 refers to vegetables. Howerver, you can use 饭 to represent meal, and 菜 to represent all the dishes except the rice.
左右 and 大概 both means "about", but 大概 is usually used in front of the center word, and 左右 usually follows the center word.
Note that, you can never use 左右 and 大概 at the same time.
