The formula for determining the final cost of an item on sale is:

#c = p - (d * p)#

Where:

#c# is the final cost - £3.60 for this problem.

#p# is the original price of the item - what we are solving for in this problem.

#d# is the discount rate - 20% for this problem. "Percent" or "%" means "out of 100" or "per 100", Therefore 20% can be written as #20/100#.

Substituting and solving for #p# gives:

#£3.60 = p - (20/100 * p)#

#£3.60 = (100/100 * p) - 20/100p#

#£3.60 = 100/100p - 20/100p#

#£3.60 = (100 - 20)/100p#

#£3.60 = 80/100p#

#color(red)(100)/color(blue)(80) * £3.60 = color(red)(100)/color(blue)(80) *80/100p#

#(£360)/color(blue)(80) = cancel(color(red)(100))/cancel(color(blue)(80)) * color(blue)(cancel(color(black)(80)))/color(red)(cancel(color(black)(100)))p#

#£4.50 = p#

Before the sale the bag of springs was worth £4.50