How do you multiply patterns with decimals

There is a line with slope 1/a going through the point (a, a²). find the equation of the line perpendicular to this line at this point. (don’t let this problem intimidate you! solve the problem as you would if i’d given you specific values. just use variables instead.)

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

In a class, there are 12 girls. if the ratio of boys to girls is 5: 4 how many boys are there in the class