一道prep的数学！！！

Five pieces of wood have an average length of 124 cm and a median length of 140 cm. What is the maximun possible length, in cm, of the shortest piece of wood?

a 90

b 100

c 110

d 120

e 130

b

suppose the five pieces of the wood are a b c d and e, and a<=b<=c<=d<=e, so c is the median equal to 140.

because the average lengh of these pieces is 124 cm ,so a+b+c+d+e=620

 to maximize a ,we supose a=b, c=d=e=140 so a=100

hi, would you please explain why you suppose a=b?  Thanks a lot!

for a is the shorest ,so a<=b the bigest value for a is b 

many thanks!!!