Answer:
35%
Explanation:
To solve this, you need to add the total amount of eaten cake and subtract it from the original cake.
So, Joseph ate 2/5, and Mary ate 1/4.
To add, you need to make them a common denominator (bottom number),
so you can multiply the first fraction by 4/1 and the second number by 5/1 in order to get the same denominator, as it readies you up to add.
Now you have 8/5 and 5/4.
The least common denominator is 20, as 5 and 4 is most close to 20 together as factors.
8/20 + 5/20 = 13/20.
Since you are looking for the leftover cake, it would be the remaining cake, so 20/20 - 13/20 = 7/20
You are using percents, so 7/20 becomes 35/100, as 7/20 can be multiplied by 5/5 to get the denominator of 100.
As a percent, it would just be 35.