Here
[tex]\\ \rm\rightarrowtail x\propto \dfrac{z}{y}[/tex]
[tex]\\ \rm\rightarrowtail x=k\dfrac{z}{y}[/tex]
Put first values
[tex]\\ \rm\rightarrowtail 10=k\dfrac{12000}{12}[/tex]
[tex]\\ \rm\rightarrowtail 10=k(1000)[/tex]
[tex]\\ \rm\rightarrowtail k=0.01[/tex]
Put for same in second part
[tex]\\ \rm\rightarrowtail x_2=k\dfrac{z_2}{y_2}[/tex]
[tex]\\ \rm\rightarrowtail x_2=(0.01)\dfrac{10000}{10}[/tex]
[tex]\\ \rm\rightarrowtail x_2=(0.01)(1000)[/tex]
[tex]\\ \rm\rightarrowtail x_2=10[/tex]
Same 10boys are needed