You CAN'T calculate that, without additional information.
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10/3
a/b=c/d =>ad=bc =>a =bc/d b =ad/c c =ad/b d =bc/a so if a+b=c+d is true => (bc/d)+(ad/c)=(ad/b)+(bc/a) => (bc2+ad2)/dc=(da2+cb2)/ab => ab(bc2+ad2)=dc(da2+cb2) and since ad=bc, => ab(adc+add) =dc(ada+adc) => abadc+abadd =dcada + dcadc => abadc-dcadc =dcada-abadd => (ab-dc)adc =(dc-ab)add ad cancels out => (ab-dc)c =(dc-ab)d => -(dc-ab)c =(dc-ab)d => -c = d so there's your answer :)
20
Not enough information
Ab and Ba are the same line because there are no endpoints to a line. Therefore, you can reverse the order of the letters. So, Cd and Dc are not the same ray because the first letter is the endpoint. So on ray Cd, point C is the endpoint and d is a point on the line coming from the endpoint. On ray Dc, D is the endpoint.