the best way to solve this problem is by doing a cross multiplication set up.
\(\frac{107}{138}* \frac{100%}{?}\)
remember numbers on one side percents on the other.
what we know:
107= the number originally thought to be the cost of expansion
138= the new estimated cost of expansion (107+31)
100%= is equal to 107 because this was our original starting number
alright now we find "?"
138*100=13800
13800÷107= 128.97 or 129%
this means that the expansion was 29% over budget