In a day care group, nine babies are five months old and 12 babies are seven months old. How many full months from now will the average age of the 21 babies first surpass 20 months old?

4.5 months a

Step-by-step explanation:

9 babies are five months old

Average age = 5 months

12 babies are seven months old.

Average age = 7 months

Currently the average age of the babies are: 5months + 7months = 12 months

21 babies = 12 months

How many full months from now will the average age of the 21 babies first surpass 20 months old

Surpassing 20 months old means above 20, = 21 months

5months + x + 7 month + x = 21months

Collect like terms

12 months + 2 x = 21 months

2x = 21 months - 12 months

2x = 9 months

2x = 9 months/2

x = 4.5 months