2x = (320:8) right, and "solve the x to get the max speed".
2x means in mathematics 2 times x when "x" is the asked number (which is stated on the sign. So 320:8 (And I see I did a little mistake in my calculations above) is 40.
so two times x is 40... then 1 times x is 20 (that's the mistake I made, I forgot to divide the outcome by 2 or I did take 640:8 confused by my earlier calculation). So that means the speed limit is 20mph (US sign so mph), which equals to 32 km/h.
Damn, I really messed it up above... Now that I write it out to explain it to you, my mistake becomes clear. Fun stuff, eh? Mathematical formulas.