How many solutions to this

7x+8y=-18, 4x-9y=-3 solve the system of equations

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?

Olga bought a new skirt that cost $20. sales tax is 5%. how much did olga pay, including sales tax? 7.