Question 347014
Let x=time required to mow the lawn if both work together
Then both work at the rate of 1/x lawn per min
boy works at the rate of 1/90 lawn per min
sis works at the rate of 1/60 lawn per min
Then both work at the rate of 1/90 + 1/60=2/180+3/180=5/180=1/36 lawn per min
So our equation to solve is
1/36=1/x  multiply each side by 36x
x=36 min----time it takes both working together

CK
in 36 min boy mows (1/90)*36=36/90=6/15 of the lawn
in 36 min girl mows (1/60)*36 =36/60 =9/15 of the lawn

6/15 + 9/15=15/15=1 (lawn that is)
Hope this helps---ptaylor