One week, when the price was lowered by 5%, sales revenue went up 13% relative to the average week at the regular price. Another week, with the price lowered by 10%, sales revenue was up 18%. The week with the lowest price (lowered by 20%) had sales revenue up 29%.

Using standard methods for linear statistical prediction, find the percent increase in sales revenue that would be expected if the price were lowered by 15%.

Find the usual measure of the strength of association between price reduction and sales revenue increase.