a = lw
For a rectangular airport runway, as you described, the formula for area is "area equals length times width".
We already have the length and the width, so we can fill those is:
1/2 = 3/4w
Now we divide both sides by 3/4 to find w and isolate the variable:
1/2 ÷ 3/4 = w
To divide fractions, just multiple by the recipricol (numerator and denominator flip):
1/2 * 4/3 = w
To multiply, multiply the numerator and denominator of each:
1 * 4 = 4
2 * 3 = 6
So, 1/2 * 4/3 = 4/6 = 2/3 = w (width)