In how many distinct ways can the letters of the word robbers be arranged?

I'm terrible at math any here is appreciated.

Zoe placed colored blocks on a scale in science class. each block weighed 0.8 ounces. the total weight of all the colored blocks was 12.8 ounces. how many blocks did zoe place on the scale? write and solve an equation to find the answer.

Given the equation f(x)=a(b)x where the initial value is 40 and the growth rate is 20% what is f(4)