Leviim Working in the Mishkan

There were three families of Levites.  After the counting in the wilderness the total number of men (30 days and over) for each family was as follows:

Gershon               7,500

Kehos                   8,600

Merari                  6,200

We are also told the number of men in each family that were of age to work in the Mishkan (30 years old to 50 years old) (Bamidbar 4:36 40, 44)

Gershon               2,630

Kehos                   2,750

Merari                  3,200

Question: How many Levites (30 days and older) that were not the right age to work in the Mishkan?

Answer:

Gershon               7,500

Kehos                   8,600

Merari                  6,200

Total                    22,300

Gershon               2,630

Kehos                   2,750

Merari                  3,200

Total                    8,580

23,300 – 8,580 = 13,720 Levites (30 days and over) who could not work in the Mishkan because of their age.