Answer: 40 / 3
Justification:
A proportion is the equality of two ratios.
Your equality is:
3y - 8 y
--------- = ---
12 5
That equality is an equation. Now you must solve for y.
These are the steps:
1) Multilpy both sides by 12 * 5
=> 5 (3y - 8) = 12y
2) Expand the parentesis (distributive property)
15y - 40 = 12y
3) Transpose 12 and - 40
15y - 12y = 40
4) Combine the like terms
3y = 40
5) Divide both sides by 3
y = 40 / 3
Answer: y = 40 / 3
