请问这里portfolio return 怎么计算的

NO.PZ202303270300007701 问题如下 (1) Whis the approximate unheeexcess return to the UniteStates–basecret manager for internationcret portfolio inx equally weighteacross the four portfolio choices, assuming no change to spreration anno fault losses occur? A.–0.257% B.–0.850% C.0.750% A is correct. We solve for the excess spresubtracting ExpecteLoss from the respective OASRecall ththe UniteStates-baseinvestor must convert the euro return to US llars using R = (1 + RF(1 + RFX) -1, so the USIG anUSHY positions comprising half the portfolio return average 0.80%, while the EUR IG anEUR HY positions return -1.314% in US llterms = (1 + ((0.65% + 0.75%)/2)) × 0.98) – 1, so -0.257% = (0.80%-1.314%)/2. 我看到助教老师的回答所以，EXR(US IG）=1.25%；EXR(US HY)=3.00%；EXR(EUR IG)=-0.873%；EXR(EUR HY)=1.18%。所以portfolio 的EXR=1/4（1.25%++3%-0.873%+1.18%）=1.14%。最终答案是1.14%是吗？

NO.PZ202303270300007701 问题如下 (1) Whis the approximate unheeexcess return to the UniteStates–basecret manager for internationcret portfolio inx equally weighteacross the four portfolio choices, assuming no change to spreration anno fault losses occur? A.–0.257% B.–0.850% C.0.750% A is correct. We solve for the excess spresubtracting ExpecteLoss from the respective OASRecall ththe UniteStates-baseinvestor must convert the euro return to US llars using R = (1 + RF(1 + RFX) -1, so the USIG anUSHY positions comprising half the portfolio return average 0.80%, while the EUR IG anEUR HY positions return -1.314% in US llterms = (1 + ((0.65% + 0.75%)/2)) × 0.98) – 1, so -0.257% = (0.80%-1.314%)/2. 老师好，听何老师讲解。本题步骤先是得到us两个债券的excess return，然后加总除以2，再计算得出eur两个债券的excess return，加总除以2再进行foreign currency转换为mestic currency，最后得到us和eur的对应本币的return，然后加总除以2。为什么要进行两步加总除以2的过程？为什么不能直接在刚开始计算的时候就除以4？不是equally weighte？

NO.PZ202303270300007701问题如下 (1) Whis the approximate unheeexcess return to the UniteStates–basecret manager for internationcret portfolio inx equally weighteacross the four portfolio choices, assuming no change to spreration anno fault losses occur? A.–0.257%B.–0.850%C.0.750% A is correct. We solve for the excess spresubtracting ExpecteLoss from the respective OASRecall ththe UniteStates-baseinvestor must convert the euro return to US llars using R = (1 + RF(1 + RFX) -1, so the USIG anUSHY positions comprising half the portfolio return average 0.80%, while the EUR IG anEUR HY positions return -1.314% in US llterms = (1 + ((0.65% + 0.75%)/2)) × 0.98) – 1, so -0.257% = (0.80%-1.314%)/2. 请教一下具体步骤 谢谢没有很看得懂解析