Answer:
If 5 red shirts are added into the box, 7 : 8 is the new ratio.
Step-by-step explanation:
The number of red shirts = 16
Let the number of black shirts = k
Now as the ratio of Red Shirts : Black Shirt = 2 : 3
By RATIO OF PROPORTIONALITY:
⇒
or, 
Hence, the number of Black Shirts in the Store = 24
Now, new number pf red shirts = 16 + 5 = 21
So, by the proportionality again, ratio is

Hence, the new ratio of Red shirts : Black Shirts = 7 : 8