Sorry, but I think you could actually use either one in your sentences, especially for #1. So if you have to choose one or the other, I would use "concentrate" for #1 and "focus" for #2 (This is one of those types of questions that "nit-pick" - finding some small grammatical or lexical difference that the tests use to "catch" people. I think these questions are unfair and make me angry)

You could use either word in either sentence. (I'm beginning to have my doubts about this Virginia Evans. If I'm not mistaken, she is Greek businesswoman who's made a fortune by pretending to be a native English speaker. She's been publishing coursebooks for years under this name. There are literally hundreds of 'Express Publishing' books with the name 'Virginia Evans' on the front. I suspect that most of them are written by amateurs. If I were you, I'd stick with a more reputable publisher. )

They are often used interchangeably, but you might think of concentrate as using your brain while focus as using your eyes I agree with Rena, "concentrate" for #1 and "focus" for #2

In my opinion, in Canada, they are interchangeable in these examples. In other regions, there may be a difference in usage. Notice the definitions below: New Oxford American Dictionary FOCUS | ˈfoʊkəs | noun (plural focuses or foci | ˈfōˌsī, -ˌkī | ) 1 the center of interest or activity: this generation has made the environment a focus of attention. • an act of CONCENTRATING interest or activity on something: our focus on the customer's requirements. CONCENTRATE | ˈkɑnsənˌtreɪt | verb 1 [no object] FOCUS one's attention or mental effort on a particular object or activity: she couldn't concentrate on the movie. • (concentrate on/upon) do or deal with (one particular thing) above all others: Luke wants to concentrate on his film career.