line one b=h*5/3
line two b*h/2=120
let's substitute b in line two with the righthand side of line one
(h*5/3)*h/2=120
h²*5/6=120
h²=(120/5)*6
h²=144
h=12
(the negative solution of the root is ignored, bc h is some distance. distances are never negative)
let's plus 12 into line two, in the place of h. this will give us b
b*12/2=120
b*6=120
b=20
h=12 and b=20 are in the right ratio (note that both are multiples of 4, 12=4*3 and 20=4*5)
and 12*20/2 = 120
everything works out just nicely
