Question

what is the value of x in the proportion x+1 over x+3 = 15 over 21?

Answer 1

\(\dfrac{x + 1}{x + 3} = \dfrac{15}{21} \)

First, reduce the right fraction. Divide both the numerator and denominator by 3.

\(\dfrac{x + 1}{x + 3} = \dfrac{5}{7} \)

Now cross multiply.

\(5(x + 3) = 7(x + 1) \)

Distribute on both sides:

\(5x + 15 = 7x + 7\)

Subtract 7x from both sides and subtract 15 from both sides.

\(-2x = -8\)

Divide both sides by -2.

\(x = 4\)