Translation:Seventy centimeters plus thirty centimeters are a meter.
Nah, it shouldn't - the subject (centimetres) is plural, so the verb is plural, regardless of the singular predicate (metre). But no biggie :)
It should! 70 + 30 is one operation, and the result of this operation IS 100. Nobody says 70+30 are 100, even tough both sides of the equation are plural.