Questions

The given letter equation implies that, 

7 C in the R = 7 Colours in a Rainbow.

Tips

It would be wise to save this question for the interviewing manager, and not for a peer/technical discussion. Nobody likes a kiss-up, but letting management know that you will communicate openly and honestly with them, always scores big points. The last part of the question can be a good barometer about how easy it will be to become a top performer. You can follow up with a discussion of how you have been successful in your previous jobs.

We will rewrite the formula as an equation.

2K + 2Cr + 7O = 0

The zero comes from there being no overall charge on the compound.

Each oxidation number is represented by the element's symbol.

Since this compound is not a peroxide or superoxide, the oxidation value for O is -2

K being in group 1 has a +1 charge and therefore has an oxidation number of K is +1

Now, by putting known values in we have:

2(+1) + 2(Cr) + 7(-2) = 0

2 + 2Cr -14 = 0

2Cr = +12

Cr = +6.