No expression has been provided, but here’s a guide to how you do it.

Let’s say you’re given an expression consisting of numbers and arithmetic operations like add, subtract, multiply, divide, etc. Or you could be given an algebraic expression containing variables and you’re told what each variable’s value is. So you substitute these values wherever the variables appear in the expression.

The first thing to do is to round the quantities to a number you can deal with more easily. For example, one of the numbers or quantities could be 11.23. You might find that changing it to 11 or 10 makes it easier for you to estimate the answer, especially if you can do the calculation in your head. The idea is to do the calculation without using a calculator.

The best estimate comes from replacing all the quantities with close substitutes so that the calculation is easier. Someone else doing the same problem might make different substitutions and their answer may be a bit different from yours, but it’s still a best estimate. In other words, the best answer you can produce without using a calculator to give you the exact answer. There will be only one exact answer but there will be many best estimates.

Sometimes when you’re using a calculator it helps if you do a quick estimate of the answer. This will tell you what to expect when you do the calculation with a calculator, or use long multiplication or long division, or whatever. Then the exact answer should not be too far away from your best estimate.