Question 335517
let work involved = 4 units,
then J & B each work @ 1 unit/hr
ie J+B = 2 
in 1 hr they do 2 units of work
2 more units remain
J+B+R finish 2 units of work in 1/2 hr, @ 4units/hr
so R works @ 2units/hr
B+R will work @ 1+2 = 3 units/hr
so they will take 4/3
= 1 1/3 h
= 1 h 20 m