I find that t is easiest to solve these types of questions when you make it into an equation.
This can be written as 250=30x+25
X is the amount of vegetable plots Arthur can plant. Since each vegetable plot is 30 square yards, we multiply the amount of vegetable plots he can place, which is unknown, by 30. Thus, it comes out 30x.
Since he already used up 25 square yards of the backyard, we write it as +25.
Now, solve.
250=30x+25
225=30x
7.5=x
Arthur can make 7 more plots (Or 7.5 of you want to be specific.
Hope this helps!