Which decimal is greater 0.9 or 0.90

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

Me i’m stuck on all these questions besides the two bottom ones

Add. express your answer in simplest form. 12 9/10+2 2/3 = i need u guys to me find the answer to this question