4.5 x 4.1= please help I'm in class

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

Solve for x and y: 217x + 131y = 913; 131x + 217y = 827

Due to bad planning, two of the local schools have their annual plays on the same weekend. while 457 people attend one, and 372 attend the other, only 104 people were able to attend both. how many people went to at least one play?