Please help me answer #2

Acomputer store bought a program at a cost of $10 and sold it for $13. find the percent markup

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?

3x = 729 solve for x plz . it's urgent. will mark