Question 1203763
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I don't know exactly what method you are calling the "formula method"....  But simple logical reasoning can find the answer with far less work than with any formal algebra.<br>
15 men working instead of 20 is 15/20 = 3/4 as many men, so to complete the job requires 4/3 as many day.<br>
4/3 of 60 days is 80 days, which is 20 days more than 60.<br>
ANSWER: 20 more days<br>