There are two ways you could answer that question:
1. The Formulaic Method
Compound Interest = (Initial Amount)*(1+ (Interest Rate/100))
2. The Easy Method
Compound Interest is named that way to signify that you are compounding or adding the interest earned every year to the initial amount.
What that means is,
If I put in $100 initially at a rate of 10% compounded annually, I would get 10% of 100 in the first year, which is $10.
Now, in the second year, my bank balance has increased to $110 because they gave me that 10$
So by the end of the second year, I would get another 10% of 110, which is $11.
This way, you can see that every year you are simply multiplying the rate by the previous year's balance.
To simplify, step 1 was 10% or 0.1*100, step 2 was 0.1*110 or 0.1*(0.1*100)
By multiplying the rate by the previous product, we get a simple formula as
Compound Interest = Initial Amount*Rate*Rate*Rate (multiply rate as many years they have mentioned)
For further details, message me. I'll teach you about compounding monthly and more.