On Friday there were 367 + 178 = 545 boys and girls who rode the Twister

On Saturday there were 165 + 218 = 383 boys and girls who rode the Twister

n = 545 - 383 = 162

If you put another rectangle box with the number 162 inside where n is, then the Saturday strip diagram will be the same length as the Friday strip diagram.

That's why B is the answer. Because it finds the difference between the total number of children on both days.