15 feet of string is 15*12=<<15*12=180>>180 inches long.
If for every 6-inch wick she also cuts a 12-inch wick, then the pair of wicks are 6+12=<<6+12=18>>18 inches long.
Thus, in 180 inches of string, she can cut 180/18=<<180/18=10>>10 pairs of wicks.
10 pairs of wicks are a total of 10*2=<<10*2=20>>20 wicks
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