A metal wire strain gauge is firmly fixed across a crack in a wall, as shown in the figure below, so that the growth of the crack may be monitored.
The strain gauge has an initial resistance of 
and, after being fixed in position across the crack for several weeks, the resistance is found to be 
. The change in the area of the cross-section of the strain gauge wire is negligible. Calculate the percentage increase in the width of the crack. Explain your working.
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