Explanation:
since we are taking about true halves (I hope), both halves "weigh" the same, and a percentage in one has the same absolute value as in the other.
then the answer is just based on the mean value between both percentages :
x = percentage of the second half
70 = (78 + x)/2
140 = 78 + x
x = 140 - 78 = 62%
she needs at least 62% on the second half to pass the 70% minimum limit of the whole test.