Answer:
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
