$$\frac{1}{15}-\frac{1}{20}=\frac{1}{60}$$,  $$1\div\frac{1}{60}=60$$. 
Eddie and Frank can complete a job together in $$15$$ days. Eddie can do it alone in $$20$$ days. Frank can do it alone indays. 

* $$30$$ 
* $$40$$ 
* $$45$$ 
* $$60$$ 