Work-rate (time and work) problem solved via combined-rate equations. If A+B finish a job in t hours, their combined rate is 1/t job/hour. Rates of pairs sum to twice the sum of individual rates when all three pairwise combinations are given.
Let rates (job/hr) be T, D, H. Given: T+D=1/2, T+H=1/3, D+H=1/6. Adding all three: 2(T+D+H)=1/2+1/3+1/6=1, so T+D+H=1/2. Subtracting each pair equation from this total: H=1/2-1/2=0,...
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