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# PHP: Composition of operations

Suppose we want to calculate an expression like `3 * 5 - 2`? This is how we'd write it down:

``````<?php

print_r(3 * 5 - 2); // => 13
``````

Note that the interpreter performs arithmetic operations in the right order: first division and multiplication, then addition and subtraction. Sometimes we want to change the order of operations. We'll dig into this topic in the next lesson.

Or another example:

``````<?php

print_r(2 * 4 * 5 * 10); // 2 * 4 * 5 * 10 = 8 * 5 * 10 = 40 * 10 => 400
``````

We can combine operations, which allows us to calculate even more complex compound expressions. This property of operations is called composition. The composition of arithmetic operations in programming is similar to how it's done in school. As you'll later learn, the composition of operations applies to all operations in general, not just arithmetic operations.

To visualize how calculations are done inside the interpreter, let's look at an example: `2 * 4 * 5 * 10`.

1. First we calculate `2 * 4` and get `8 * 5 * 10`.
2. Then `8 * 5`. The result is `40 * 10`.
3. Finally, we do the last multiplication, and the result is `400`.

## Instructions

Write a program that calculates and prints the value of this expression:

``````8 / 2 + 5 - -3 / 2
``````

Don't calculate anything manually, your program should do all the calculations on its own.

The exercise doesn't pass checking. What to do? 😶

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In my environment the code works, but not here 🤨

Tests are designed so that they test the solution in different ways and against different data. Often the solution works with one kind of input data but doesn't work with others. Check the «Tests» tab to figure this out, you can find hints at the error output.

My code is different from the teacher's one 🤔

It's fine. 🙆 One task in programming can be solved in many different ways. If your code passed all tests, it complies with the task conditions.

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