Needexplain how to get the answer

What is the definition of exponential decay?

Identify the initial amount a and the growth factor b in the exponential function. a(x)=680*4.3^x

Money off coupons have been circulated to 300 households. only 2/5 of these were redeemed (used) in the local supermarket to get a free shampoo. what fraction of coupons were unused? (ps: write how you got the answer)