I don’t understand exponential decay

Decay means the value gets smaller.

In order for a value to get smaller you need to multiply by a value less than 1

The first equation uses 0.5 which is less than 1

The second equation uses 0.4 which is less than 1

The third equation does not have a decimal and uses 4, which is greater than 1, so cannot get smaller.

The fourth equation uses 0.5 which is less than 1

The answer would be the 3rd one.

step-by-step explanation:

(s)he  is right it would be 5.5% because it is smaller than 15.5% or anything smaller than 15.5% in general

55? i think maybe

step-by-step explanation: