What does a daily drink cost you by graduation?

The question

What does a daily drink cost you by graduation?

Before you start

“Do you use Chromebooks in this class?” Ask the room. If the answer is yes and devices are in the room, run the Sheets version. Otherwise, run it on paper. Both versions take about the same time.

Set up the grid

Also write on the board: 5 school days a week. About 180 school days a year. Years left: ___. Have the room agree on the number of years left. For a mixed class, pick one number.

Tell them: “180 is an assumption. Your school year might be a little different. That’s fine.”

Build it

1. Per week = price × 5. Sheets: =B2*5
2. Per school year = price × 180. Point out that this is not per week × 180, since that counts each day five times. Sheets: =B2*180
3. Through graduation = per school year × years left. Sheets: =D2*4, or whatever the room agreed on.
4. Fill the other two rows. On paper, students work out each row. In Sheets, students fill down.
5. Add a total row under the last column. Sheets: =SUM(E2:E4)

The answer

With four years left, the results are roughly:

Sanity check: per week × 36 should land on the same per-year number, since 36 weeks × 5 days = 180 days. Two routes to one answer means the grid is right.

Where they get stuck

• Per week × 180. The number comes out five times too big. Ask: “Does anyone drink five of these a day?”
• Sheets shows the formula instead of a number. The formula is missing the =.

Close

“If you wanted to change the answer the most, would you change the price or the number of days?” Let two or three students answer. Then run it: halving the days and halving the price change the result by the same amount. Some students will be surprised by that, and the surprise is the point.

In the note