General Idea:
When we are working with word problems, we need to follow the below steps:
Step 1: Assign variable for the unknown that we need to find.
Step 2: Write a meaningful mathematical equation using the sentence given
Step 3: Solve the equation by Performing reverse operation by Undoing whatever is done to the variable. Solving means find the value of the variable which will make the equation TRUE.
Applying the concept:
Step 1: Let x be the number of books that Brett can afford without spending more than his gift voucher amount.
Step 2: Writing mathematical equation based on the given sentence "Brett has a $30 online gift voucher. He plans to buy as many books as he can. The cost of each book is $4. There is also a single shipping charge of $2"


Step 3: Solving equation

Conclusion:
The number of books that Brett can afford without spending more than his gift voucher amount is 7 !!