$420-\left( 55\times5\right)=420-275=145$ 
A train travels from Exeter to Cambridge at a~ speed of $$55km/h$$. Given that Cambridge is $$420km$$ away from Exeter, How far is the train away from Cambridge after setting off for 5 hours? 
Options:
[a] $$55km$$ 
[b] $$145km$$ 
[c] $$275km$$ 
[d] $$420km$$ 