job done = time x rate
let job done = 1, since it is a constant
John's rate --- 1/12
Mike's rate ---- 1/x
combined rate = 1/12 + 1/x = (x+12)/(12x)
after John worked for 3 hours
job done = 3(1/12) = 1/4
so job left to be done = 3/4
(3/4) / ((x+12)/(12x)) = 5
(3/4)(12x)/(x+12) = 5
36x/(x+12) = 20
36x = 20x + 240
x = 15
Alone, Mike could do it in 15 hours
check my arithmetic
It takes john 12 hours to plow a field. After he had been working for 3 hours, his brother Mike came to help. Together they finished the job in 5 hours. How long would it take mike to plow the field working alone?
How would I set this problem up in fraction form? And would I need to multiply it out soon afterword?
1 answer