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To summarize:

If one quantity increases (decreases) as another quantity also increases (decreases), set ratios equal. If one quantity increases (decreases) as another quantity decreases (increases), set products equal.

The concept of proportion can be generalized to three or more ratios. A, B, and C are in the ratio 3:4:5 means $\frac{A}{B}$ = $\frac{3}{4}$, $\frac{A}{C}$ = $\frac{3}{5}$, and $\frac{B}{C}$ = $\frac{4}{5}$.

Example:
In the figure to the right, the angles A, B, C of the triangle are in the ratio 5:12:13. What is the measure of angle A?

(A) 15
(B) 27
(C) 30
(D) 34
(E) 40

Since the angle sum of a triangle is 180Â°, A + B + C = 180. Forming two of the ratios yields
$\frac{A}{B}$ = $\frac{5}{12}$     $\frac{A}{C}$ = $\frac{5}{13}$

Solving the first equation for B yields
B = $\frac{12}{5}$A

Solving the second equation for C yields
C = $\frac{13}{5}$A

Hence, 180 = A + B + C = A + $\frac{12}{5}$A + $\frac{13}{5}$A = 6A . Therefore, 180 = 6A, or A = 30. The answer is choice (C).