AutoSum, AVERAGE, and checking that the answer makes sense

When numbers are already in a column or a row, Microsoft offers AutoSum so you do not have to type =B2+B3+B4+... by hand. The basic-tasks page says:

1. Select the cell to the right of, or below, the numbers you want to add.
2. Click the Home tab, and then click AutoSum in the Editing group.
3. AutoSum adds the numbers and shows the result in the cell you selected.

The simple-formula page gives the same idea and adds that AutoSum is also on the Formulas tab. When you select AutoSum, Excel enters a formula that uses the SUM function and highlights the cells it plans to total. You press Enter (Windows) or Return (Mac) to accept. On Windows, Microsoft also lists the shortcut Alt+= , which inserts SUM.

To total a column, select the cell immediately below the last number. To total a row, select the cell immediately to the right. On the Hours sheet, that means clicking B9 (under Sunday's hours) and using AutoSum. With the sample data, Excel should propose something like =SUM(B2:B8). After you press Enter, the result is 25.5 hours (6+4+7.5+0+5+3+0).

Look at the proposed range before you press Enter. If AutoSum highlighted the wrong cells — for example if it included a label — drag to correct the selection or edit the range in the formula bar, then press Enter. The colon in B2:B8 means “from B2 through B8.”

Excel can also calculate an average (the mean): add the numbers, then divide by how many numbers were included. Microsoft's “Calculate an average” page shows this path:

1. Select the cells you want to average, *or* select the cell where the average should appear.
2. On the Formulas tab, select the arrow next to AutoSum, and then select Average.

When you create the average in its own cell, the formula bar shows a function that looks like =AVERAGE(A2:A7) in Microsoft's sample. On the Hours sheet, click B11 (leaving a blank row under the total is fine) or another empty cell, choose Formulas > AutoSum arrow > Average, and check that the range points at the hours, B2:B8, not at the total in B9. If AutoSum's average includes the total, the answer will be too high. Edit the range so it covers only the days.

With the sample hours 6, 4, 7.5, 0, 5, 3, and 0, the average is 25.5 ÷ 7, which is about 3.64 hours per day if every listed day counts — including Thursday and Sunday as zeros. That is mathematically correct for “average of the seven recorded days.” It is not the same as “average of only the days I worked.” If you need the second meaning, you would average only the working days (or leave days off out of the sheet). This course does not teach extra functions that ignore certain values. What it teaches is to read the range inside AVERAGE and know which cells are included.

You can also type =AVERAGE(B2:B8) yourself, the same way you typed =SUM(B2:B8). Starting with = and using a colon range is the same habit as Lesson 5, with a function name added. Microsoft's overview of formulas shows that pattern: type =, a function name, (, the range, then ).

A formula will calculate exactly what you asked — even if you asked for the wrong range. Skills for Success describe Numeracy as including interpreting and evaluating information, and they give this kind of check: see whether the answer is of the expected size. That habit belongs beside AutoSum and AVERAGE, not after a mistake has already been shared.

Before you accept 25.5 as the week's hours, estimate. Monday through Saturday without a calculator is roughly 6 + 4 + 8 + 0 + 5 + 3 + 0, which is about 26. 25.5 is close, so it is plausible. If AutoSum showed 255 or 7.5, something would be wrong: a decimal in the wrong place, a range that grabbed the wrong cells, or a number typed as text.

Checks that catch many everyday mistakes:

• Read the formula bar. Does =SUM(B2:B8) start at the first day and stop before the total cell?
• Compare SUM to a rough mental total.
• Remember that zeros count in both SUM and AVERAGE. They do not change the sum, but they do pull an average down.
• If a total looks like a date, the cell may have the wrong number format (Lesson 4). Set it back to Number or General.
• If SUM ignores a number, that number may have been typed as text (6 hours). Type a plain number instead.

Do not type over a formula result with a number you calculated by hand unless you intend to remove the formula. If you type 25.5 into B9, the cell will no longer update when Friday's hours change.

1. Put =SUM(B2:B8) in Hours B9 (AutoSum is the fastest path) and confirm 25.5.
2. Put =SUM(C2:C6) in Household C7 and confirm 158.78.
3. Put =AVERAGE(B2:B8) in an empty Hours cell and confirm a value near 3.64.
4. Change Friday (B6) from 5 to 5.5 and watch the total update to 26. Then put B6 back to 5 so the sample file stays correct.
5. Read each formula in the formula bar and say out loud which cells it includes.

When would you report a week's average including days off, and when would that number be misleading? Who needs to know which days you included?