Using a decimal addition table for subtracting

Question

I'm reviewing the math I missed in school and I've come across a decimal addition table in a book along with a description of how to use the same table for subtracting. I'm having trouble parsing the description on how to subtract, though.

"The decimal addition table can also be used for subtraction! This is because subtraction is the inverse of addition. For example, to solve $9-7$, find the $7$ in the top row and drop down to the nine in the table. The difference, 2, is in the same row as nine on the left side of the table."

I'm overthinking it. It's a habit. Here's an image of the table:

table

Because you are finding the $?$ which fits $7+? = 9$. – peterwhy Dec 20 '14 at 18:05 — peterwhy, Dec 20 '14 at 18:05

score 1 · Accepted Answer · answered Dec 20 '14 at 20:48

1

Sometimes a diagram may help. Here is the table from your source with circles and arrows (but no paragraph on the back of it) showing the steps of the calculation.

enter image description here

When they say "drop down" they mean follow the downward-pointing arrow, and for "the same row as nine" they intend you to follow the left-pointing arrow.

answered Dec 20 '14 at 20:48

David K

98,388

Wonderful! Thank you much!! – tcv Dec 21 '14 at 17:16

score 0 · Answer 2 · answered Dec 20 '14 at 18:09

0

You want to compute $ x = 9 - 7 $, but this is the same as finding $x$ that satisfies $x +7 = 9$.

answered Dec 20 '14 at 18:09

fritzenbauer

186

score 0 · Answer 3 · answered Dec 20 '14 at 18:19

For adding

Sum = column of the first no.+ row of the second no.

Sum - column of the first no. = row of the second no.

For 9-7

So the large no. is the sum 9, small number is the column number, and corresponding row no. is the subtraction (column no. , row no. are equal to the first row and first columns respectively. )

Using a decimal addition table for subtracting

3 Answers3