`1/120 - 1/300 = 5/600 - 2/600 = 3/600 = 1/200`
\(\dfrac{1}{120}-\dfrac{1}{300}=\dfrac{300}{36000}-\dfrac{120}{36000}=\dfrac{180}{36000}=\dfrac{1}{200}\)
\(\dfrac{1}{120}-\dfrac{1}{300}=\dfrac{1\times300}{120\times300}-\dfrac{1\times120}{300\times120}=\dfrac{300}{36000}-\dfrac{120}{36000}=\dfrac{300-120}{36000}=\dfrac{180}{36000}=\dfrac{1}{200}.\)