Find tax percent given tax amount and total amount

Question

First, I will start with example ...

for example meal price is : 105 

and meal total price + the tip is : 120

so left with : 15

how I can tell how much % the 15 is from the 105 (meal price)?

Answer 1

The "percent" one number is of another is the equivalent number of parts per one hundred.

So, for example, since $\dfrac{1}{4} = \dfrac{\boxed{25}}{100}$, you know $1$ is $25\%$ of $4$. To compute that, you would be solving $\dfrac 14 = \dfrac p{100}$ for $p$.

In your specific case, you want to solve $$\frac{15}{105}=\frac{p}{100}$$ You then obtain $$p = \frac{100\cdot 15}{105}\approx 14.3,$$ that is, $15$ is approximately $14.3\%$ of $105$.