m1=0.3 кг m2=0.2 кг t1= -10 t2=0 t3=10 c1=2100 c2=4200 L=3.4*10^5
m3=?
===
c2*m3*(t3-t2)=c1*m1*(t2-t1)+L*(m1-m2)
m3=(c1*m1*(t2-t1)+L*(m1-m2))/(c2*(t3-t2))=
=(2100*0.3*10+3.4*10^5*(0.3-0.2))/(4200*10)=0.96 кг (≈1 кг)
=======================================