3    5    13    19
Step1.  3 & 5 together (elapsed time 3 minutes)

3    2/3    13    19
Step2.  3 & 13 together (elapsed time 6 minutes)

3    2/3    3/10    19
Step3.  10 & 19 together (elapsed time 16 minutes)

3    2/3    13    9/10
Step4.  2 & 9 together (elapsed time 18 minutes)

3    5    13    7/12
Step5.  Begin cooking & start 13 (elapsed time 31 minutes)

3    5    13    7/12
Step6.  start 7 (elapsed time 38 minutes)

3    5    13    19
Step7.  Finish cooking 38 - 18 = 20