How do you estimate a family spending on food if you assume that the spending on food increases linearly and a family spent $3600 on food last year and $3000 the year before the last?
Linear growth means that the value grows by the same amount in each time period.
If the family spent $3000 two years ago, and $3600 one year ago, the growth between those years was:
$3600 - $3000 = $600
If food spending increases linearly, then the growth from one year ago to this year would once again be the same amount, $600.
Therefore, the spending on food this year would be:
$3600 + $600 = $4200