Since there are 1500 people in your random sample, and the statistics show that 19.9% of people in that bracket smoke, convert your decimal to a percent (divide by 100, so you would get 0.199) and multiply that by the number in your sample, 1500. This gives you 298.5. You cannot have a fraction of a person, so round up to 299 people.
Quadratic formula is derived from completing the square:
<span>ax² + bx + c = 0 </span>
<span>ax² + bx = −c </span>
<span>x² + b/a x = −c/a </span>
<span>Complete square on left side by adding (b/(2a))² to both sides: </span>
<span>x² + b/a x + (b/(2a))² = (b/(2a))² − c/a </span>
<span>(x + b/(2a))² = (b²−4ac)/(2a)² </span>
<span>x + b/(2a) = ± √(b²−4ac)/(2a) </span>
<span>x = −b/(2a) ± √(b²−4ac)/(2a) </span>
<span>x = (−b ± √(b²−4ac)) / (2a) </span>
<span>- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - </span>
<span>or </span>
<span>ax² + bx + c = 0 </span>
<span>ax² + bx = −c </span>
<span>4a (ax² + bx) = −4ac </span>
<span>4a²x² + 4abx = −4ac </span>
<span>Complete the square on left side by adding b² to both sides </span>
<span>4a²x² + 4abx + b² = b²−4ac </span>
<span>(2ax + b)² = b²−4ac </span>
<span>2ax + b = ± √(b²−4ac) </span>
<span>2ax = −b ± √(b²−4ac) </span>
<span>x = (−b ± √(b²−4ac)) / (2a)</span>
If you wanted to figure out how much Jonathan would get in multiple weeks then multiply 18 by the number of weeks to get the total amount of money Jonathan receives for allowance during the weeks.