__Keep in mind you have to consider__. Let's see how the model program could be intepreted

`(n-1)`

as upper limitC

if (n <= 2) return 0;This is trivial, since

`1`

is not divisible by `2`

or `3`

.C

if (n == 3) return 2;This is trivial too, since in the

`1,2`

sequence, there is only `2`

which is divisible.C

int twoSum = (n - 1) % 2 == 0? (n - 1): 0;This means

C

int twoSum; if ( (n-1) % 2 == 0) twoSum = (n-1); else twoSum = 0;

That is

*"if (n-1) is divisible by*.

`2`

then keep it in order to add it to the sum"The intepretation of

C

int threeSum = (n - 1) % 3 == 0? (n - 1): 0;is similar.

C

return twoSum + threeSum + threePlusTwo(n - 1);This makes the trick of adding two times a number which divisible both by

`2`

and `3`

(please note such a requirement is not obvious from the question title).