What makes composing functions different from doing an arithmetic operation of two functions such as adding or dividing them?

What is 20+(5 x 2/5 )+ 3 show your work

12. the amount of money in a bank account decreased by 21.5% over the last year. if the amount of money at the beginning of the year is represented by ‘n', write 3 equivalent expressions to represent the amount of money in the bank account after the decrease?

Let f(x) = 5/x and g(x)=2x2+5x. what two numbers are not in the domain of f o g