A good answer might be:

 ( a > b && 45 <= sum  ) ||  (sum < a + b && d > 90 )

Expressions and Subexpressions

a + bis an arithmetic expression
sum < a + bis a relational expression,
a type of boolean expression
d > 90is a relational expression,
a type of boolean expression
sum < a + b && d > 90is a boolean expression
... and so on ... There are many other subexpressions

The boolean expression of the question contained pieces, called sub-expressions, that are themselves expressions. It is common in Java (and other languages) to build up complicated expressions out of smaller expressions. The precedence rules and parentheses keep everything straight.

Arithmetic operators *, /, +, -, and % have higher precedence than && and || so they are done first. For example: 

sum < a + b

means 

sum < (a + b)

However, the not operator (!) has higher precedence than all the arithmetic operators except unary minus and has higher precedence than the other logical operators. Usually this means that you will need to use parentheses to apply it correctly.

QUESTION 17:

People may enter a contest UNLESS they are less than 21 years old, make more than $100,000, or live in Ohio. Use parentheses to group this boolean expression so that it is true when a person CAN enter the contest. 

! age < 21 || pay > 100000 || home.equals("Ohio")