I found one formula:

M1 * x1+ m2 * x2=x3(m1+m2)

m - mass of substance

x - concentration in percent

x3 - final percentage

So in this question m1 is unknown, as NH3 consists of only NH3 x1=100%, m2=200g x2=10%,

x3=15%. from this I GOT answer 11,7 g which means 0,69 moles of NH3 as 1 mole equals to 22,4

litres I assumed that answer will be {15.456} and it is incorrect.

I have solved many exercises in this way and they were correct.