Level
pending

The answer is 15.

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Let x,y and z be work done per day by A,B and C respectively and W be the total work. W=20x W=30y W=60z Then let n be no. days in which A will do the work with help of B and C on every third day W=n(x)+(n/3)(y+z) W/n=(W/20)+(W/90)+(W/180) W/n=12W/180 n=180/12=15