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The chances that it would rain are 20%.

A value that measures the chances of occurrence of a specific event is termed as the probability corresponding to the event.

For any event E, the probability can be mathematically formulated as:

$P\left(E\right)=\frac{\text{Number}\text{of}\text{favourable}\text{outcomes}}{\text{Total}\text{number}\text{of}\text{outcomes}}$

The value of a defined event always lies between 0 to 1.

The probability value can be obtained from the percentage value by dividing it by 100.

The probability of the event and its complement is always 1.

$P\left(E\right)+P\left(\stackrel{炉}{E}\right)=1$

Consider event A as the event that it rains tomorrow.

Complementary to this, the event that it does not rain tomorrow is satisfied as the occurrence of rain may or may not occur.

Thus, $\stackrel{炉}{A}$ is defined as the event that there will be no rain tomorrow.

Using the properties, the probability that there will be no rain tomorrow is given as:

$$\begin{gathered} P\left(\stackrel{炉}{A}\right)=1-P\left(A\right) \\ =1-0.20 \\ =0.8 \end{gathered}$$

Thus, the probability of no rain tomorrow is 0.8.