You can set up a ratio problem
take what you know and set it equal to what you don't know
4/6 = r/15
Cross multiply and get:
60 = 6r
divide by 6
r = 10
3(x-7)=4•6
3x-21=24
3x=45
X=15
The population after 3 years would be 8.20 millions.
Step-by-step explanation:
Given,
Initial population = I = 4.2 million
Growing rate = r = 25%
If the initial population and growing rate is given, the formula to calculate the population (x) after n years would be:

By putting the values in the equation, we get

The population after 1st year (x1) would be

x1 = 5.25 million
Similarly,

x2 = 6.56 million

x3 = 8.20 million
Hence, the population after 3 years would be 8.20 millions.
But, if you are asking about the total population at that time. It would be
I + x1 + x2 + x3 = 24.21 million