In this activity, students perform decimal operations and estimate with money in a real-world context. They are asked to plan a dinner party for 8 guests with a $50 budget for food and drinks. Students use an actual grocery store price list, select the foods they wish to serve, and determine an appropriate amount of each item. Along the way, they have opportunities to make assumptions as they model with mathematics (MP4) and to decide on a level of precision in which to communicate the values used in their budget (MP6).

To determine the quantities and costs of menu items, students need to make choices and apply what they know about reasoning with decimals, such as:

• Round decimals to estimate costs.
• Divide to determine unit costs (per item or per guest).
• Multiply and divide to find subtotal and total costs.
• Subtract as they remove items to cut costs (if they go over budget).

Students are likely to check if their choices are within budget in two ways: by comparing their estimated total costs to 50, or by comparing cost per guest to $6.25 (which is ). As students work, monitor for students who use each approach.

If needed, consider adjusting the budget based on local costs of living. Calculators should not be used in this lesson.

The goal of this discussion is to highlight the decimal operations and estimations that students did while planning the dinner party. Select a couple of groups to share their menus with the entire class. Then, discuss questions such as:

• “For each menu item, how did you find the estimated cost per person?” (Divided the estimated subtotal by 8. Used the unit price, if each item was for 1 person.)
• “How did you determine if your menu choices are within budget? Did you look at total estimated cost, or estimated cost per person? Why?” (Total estimated cost, because the estimated subtotals were whole numbers and easier to add. Cost per person, because the numbers were smaller and easier to add.)
• “Were there items where it was difficult to estimate the cost per person? How so?” (Some bulk items are packaged to serve far more than 8 people, so it was unclear how to find the cost per person. Some numbers were harder to divide by 8.)
• “How did you find the actual cost of each of the two most expensive items?” (Multiplied the quantity needed by the advertised price.)
• “Did you know how your estimated total would compare to the total from adding all the exact costs? Would it be higher or lower?” (If the estimates were made by rounding up the exact costs, then the exact total cost would be less than the estimate. If the estimates were made by rounding down, then the exact cost would be higher. If there were a mix of rounding up and down, it would be a little harder to predict.)