Hey there!
To find the markup percentage, you must use your formula for percent markup:
profit/cost*100
Because your profit is found by subtracting 13-10, 3, your profit would be 3. You also know that your cost is 10.
Now, plug in your values:
3/10*100
And simplify:
3/10*100
0.3*100
=30
Therefore, your percent markup would be about 30%.
Hope this helps and have a nice day!
