Question 193115
let  x  =  total  job,,,  sq  ft  etc

jacks  rate  is  ,,,,x/7,,,,Sues rate  is  x/4

rate  *time  =  job

x/7*t+x/4*t=x

mult  by  28,,,,divide  by  x

4t+7t=28

11t=28

t=28/11=2.55  hrs   ANSWER

check
x/7 *2.55+x/4*2.55=x
.364x +.636x =x
x=x   ok