4200*0.5*(90-T)=400*0.2(T-20)

Solution for 4200*0.5*(90-T)=400*0.2(T-20) equation:

4200*0.5*(90-t)=400*0.2(t-20)

We simplify the equation to the form, which is simple to understand 
4200*0.5*(90-t)=400*0.2(t-20)

Simplifying:
+2100*(90-t)=400*0.2*(t-20)

Simplifying:
+2100*(90-t)=+80*(t-20)

Reorder the terms in parentheses
+(+189000-2100t)=+80*(t-20)

Remove unnecessary parentheses
+189000-2100t=+80+*(+t-20+)

Reorder the terms in parentheses
+189000-2100t=+(+80t-1600)

Remove unnecessary parentheses
+189000-2100t=+80t-1600

We move all terms containing t to the left and all other terms to the right. 
-2100t-80t=-1600-189000

We simplify left and right side of the equation. 
-2180t=-190600

We divide both sides of the equation by -2180 to get t. 
t=87.4311926606