exponentials are a way of doing lot's of multiplications or divisions really quickly.
For example, I could write 5*5*5*5*5*5*5*5, or I could just do 58.
The exponent is the number of times 1 is being multiplied by the base (in my example, 5).
A 1-time payment does not need an exponential, because you're only multiplying once.
In fact, a recurring payment (like getting $5 per day) doesn't need an exponent either, because you would add those together, not multiply.
Exponents are used when growth builds on itself.
A) the money you make per child never goes up. You would write this as y = 10x, where x is the number of children.
B) Imagine you start out with 1 bacteria. Then you have 3, then 9, then 27, then 81, 243, etc... This would be modeled by y = 3x, because each out you are multiplying whatever you have by 3, then multiplying THAT by 3, and on and on. X is the number of times you multiply by 3.
C) This is just like the last one, but instead of multiplying by 3, you're dividing by 2. dividing by 2 is the same thing as multiplying by 1/2, so we can model this one as y = (1/2)x. Each day (x), we are multiplying the amount of water left by 1/2. So x is the number of times we multiply by 1/2.
D) to find a percentage, we multiply the salary by 0.11. However, this is a 1-time payment, so we do not need to use an exponent.