问题如下：
Maud, an analyst of an investment company, she made two scenarios of portfolio returns under different economic situations:
The expected portfolio return is closest to:
选项：
A.14.0%
B.10.2%.
C.5.2%
解释：
Good economic situation scenario: The expected return = 20%×50%+10%×50%=15%
Bad economic situation scenario: The expected return=5%×60%+(-10%)×40%= -1%
The general expected return=70%×15%+30%×(-1%)=10.2%.
老师，我列的式子是这样的，0.7*0.2*0.5+0.7*0.1*0.5+0.3*0.05*0.6+0.3*（-0.1）*0.4
这样计算和答案确实不符，但不明白为什么不可以这样计算？70%那部分其实和最后的是相符的，就是后面30%部分按这个方法与答案不符，我猜想应该是先计算概率再乘以30%有扩大负数的原因，但不明白为什么不能按这个式子算？
