A rational number are numbers that can be expressed as as fraction. They can be expressed as a ratio of two integers. An irrational is quite the opposite. An irrational number cannot be expressed as a ratio of two integers.
Taking square root of two as an example;
√2 cannot be expressed as a ratio of two integers because the result will always be a decimal. If expressed as √2/1, it is still not a rational number because of the square root of 2 at the numerator. Square root of 2 is not an integer even though 1 is an integer.
Mark is wrong because √2 is irrational and it is irrational because it cannot be expressed as a ratio of two integers <em>not due to the fact that he can write it as a fraction.</em>
