9. The number of gallons of paint needed to cover the exterior of a house with one coat of paint is estimated by the formula \( [10 n(1+w)-9 n]+350 \), where \( n \) is the number of stories, I is the length of the house in feet, and \( w \) is the width of the house in feet. Approximately how many gallon cans of paint should somebody buy in order to paint one coat on the exterior of a \( 30-\times 50 \)-foot 2 -story house?

To find out how many gallons of paint are needed for a 30-by-50-foot, 2-story house, we'll substitute \( n = 2 \), \( l = 30 \), and \( w = 50 \) into the formula \( [10 n(1+w)-9 n]+350 \). First, calculate the width term: \( 1 + w = 1 + 50 = 51 \). Then, plug in the values: \[ [10 \cdot 2 \cdot 51 - 9 \cdot 2] + 350 \] \[ = [1020 - 18] + 350 \] \[ = 1002 + 350 = 1352 \] So, approximately 1352 gallons of paint are needed for just a single coat on that house. However, since paint often comes in gallon cans, they'll need to buy those whole cans—so round up if it’s not a whole number! Here's a fun tip: Always grab an extra gallon or two! You never know when a pebble might decide to ambush your wall, or if you might want to touch up those weekends after painting!