Fred can mow a lawn in 60 minutes. Bullwinkle can mow the same lawn in 90 minutes. How long does it take for both Fred and Bullwinkle to mow the lawn if they are working together?

Let x=time required to mow the lawn if both work together

Then both work at the rate of 1/x lawn per min
fred  works at the rate of 1/60 x 

Bullwinkle at the rate of 1/90 x 

1/60  + 1/90 = 3/180   + 2/180 = 5/180  = 1/36 


rate of 1/x lawn per min so 1/36 =1/x

x= 36 min

fred mow  1/60 x  = 1/60x36 =36/60= 9/15 of the lawn 

Bullwinkle mow 1/90 x 36 = 36/90 =  6/15 of the lawn