## Explanation

### Understanding Rational and Irrational Numbers

To determine if a number like 90.790170 is irrational or not, we need to recall the definitions of rational and irrational numbers:

### Rational Numbers

A **rational number** is any number that can be expressed as the **fraction** of two integers, where the denominator is not zero. In other words, a number $r$ is rational if there exist integers $p$ and $q \neq 0$ such that:

### Irrational Numbers

An **irrational number** is a number that **cannot** be written as a simple fraction of two integers. These numbers have **non-repeating, non-terminating decimal** expansions.

### Analysis of the Given Number

The number 90.790170 has a **finite decimal representation**. Since it stops after a certain number of decimal places, it can be expressed as a fraction of two integers.

### Fraction Representation

To express 90.790170 as a fraction, consider the following conversion steps:

**Write down the number without the decimal point:**

$90790170$

**Determine the place value of the last digit:**

Since there are 6 digits after the decimal point, the number can be written as:

$90.790170 = \frac{90790170}{1000000}$

**Simplify the fraction**(if possible) to find the simplest form:

If we simplify, we find:

$\frac{90790170}{1000000} = \frac{9079017}{100000}$Thus, **90.790170 is a rational number**.

### Conclusion

Given that 90.790170 has a finite decimal representation and can be expressed as a fraction of two integers, it is undoubtedly a **rational number**.