Can anyone check my answers to see if it’s correct?

Two sides of a triangle mesure 20cm and 30cm. what is the measure of the third side

Suppose that sahil knows that 45 people with ages of 18 to 29 voted. without using a calculator, he quickly says then 135 people with ages of 30to 49 voted. is he correct? how might sohil have come up with his answer so quickly?

Which is the graph of the following inequality?