St (1)- M=20 -Gives us no information of Jack or of R (Insufficient)
St(2)- R=10 -Gives us no information of Jack. Mac- (Clearly Insufficient)
We know M=20 and Jack alone completed the work in T+10 days.
=>Total work = Work completed by THEM in T days + Work by Jack in 10 days
=> T*(1/20+1/J)+ 10*1/J = 1
Its also provided in the Q-stem that they completed an equal amount of work.
T * 1/20 = T*1/J + 10 * 1/J
=>T and J can be computed .(Sufficient)
Since they complete equal amount of work,
T/M = 1/2 AND T+R/J=1/2
On combining we have,
=> J=20+2*10 =40 (Sufficient)
It is essential to point out here that the computations are all not necessary but the possibility that it can be computed is crucial to mark the correct answer choice in this DS question.
Hope that answers your query.