Totals and Differences

A juice stand records its sales in a two-way table, with a row for each day and a column for each flavour:

Every number sits where a row meets a column. Tuesday's orange sales are where the Tue row meets the Orange column: 15. A column total adds down a column, so the stand sold $12+15+9$, which is 36 orange juices. A row total adds across a row: on Monday it sold $7+12$, 19 drinks.

How many more orange than apple juices sold over the three days? How many more is a difference, so total each column, then subtract the smaller total from the larger: $\left(12+15+9\right)-\left(7+10+8\right)$ is $36-25$, which is 11. Totalling first keeps it to one subtraction at the end.