Use ratio ---> duds/firecrackers.
The first ratio is 5 duds to 136 firecracks (5/136). It must equal the ratio of duds (we don't know, so call this x) to 2720 firecrackers (x/2720).
5/136=x/2720 (Cross multiply).
136x=5*2720--->136x=13600 (divide both sides by 136)
x=100
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