Once we make an assignment, the variable takes that value.

`a = a + a * b`

The expression on the right side is evaluated, then assigned to the variable on the left. The old value is replaced.

As math goes, precedence, or order of operations apply.

```
multiplication
addition
```

So the first operation is, `a * b`

. The second operation is `a`

plus the outcome of the first operation.

Now getting a little more technical, the right side expression is a binomial, and we know that multiplication is distributed over all terms in a polynomial, which in reverse, lets us factor out the `a`

`a * (1 + b)`

which means the lesson expression can be simplified to,

`meal = meal * (1 + tax)`

The general rule of thought is to ignore the left side until all the operations are complete on the right side. If we were to create a new variable (so we could keep track of the price) it would take the result of the expression. But if we don't need to keep track, then it is better to *re-use* variables than create new ones, which is the case here.

Compare your expression to the one above. As it is calculated for this exercise, the tip is calculated on the tax, also, so,

```
meal = meal * (1 + tax)
meal = meal * (1 + tip)
```

where both `tax`

and `tip`

are decimal fractions, `0.0675`

and, `0.15`

.

Note the important role that the `1`

plays. Multiplying by 1 (unity) has no effect other than to preserve the original value. If we calculated `meal * tip`

we would end up with only the tip. Adding 1 lets us increase the meal cost to the new value.