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When faced with a proportion, which is an equation that sets two ratios equal to each other, cross multiplication is often the tool you need to solve the problem. For instance, if we have a proportion such as \(\frac{15}{45} = \frac{72}{n}\), cross multiplication helps us to find the unknown variable.

During cross multiplication, we multiply the numerator of one ratio by the denominator of the other, and set it equal to the product of the other numerator and denominator. So, with our example, the cross multiplication will look like this: \(15 \times n = 45 \times 72\). This simplifies our problem to a basic algebraic equation where we need to solve for \(n\).

Once we've cross multiplied, we isolate the variable by dividing both sides of the equation by the coefficient of the variable. In this case:\[n = \frac{45 \times 72}{15}\]. Solving this will give us the value of \(n\) that makes the original proportion true.

When rounding decimals, we look at the digit immediately to the right of the decimal place to which we are rounding. If this digit is 5 or greater, we increase the targeted decimal place by 1. If it is less than 5, the targeted decimal place remains the same.

For example, after computing \(\frac{45 * 72}{15}\) you might get an answer like 216.00. Since we're asked to round to the nearest hundredth, our answer, in this case, would just remain 216.00. However, if we had an answer like 216.275 and we need to round it to the nearest hundredth, we would look at the third decimal place. Since it is 5 or greater, we would round up the second decimal place from 7 to 8, making it 216.28.

Evaluating expressions involves performing the operations within them in the correct order, following the Order of Operations (PEMDAS - Parentheses, Exponents, Multiplication and Division, Addition and Subtraction).

Once we have isolated the variable in a problem, as we did with \(n = \frac{45 \times 72}{15}\), the next step is to perform the operations to find the value of \(n\). We multiply 45 times 72 to get 3240. Then we divide 3240 by 15, which equals 216. So, the expression evaluates to \(n = 216\).

This result needs to be interpreted in the context of the problem, and if required, rounded to the specified decimal place as instructed in the problem statement. Remember that correctly evaluating expressions requires careful calculation and, when directed, appropriate rounding.