You need a sum of 150% on both math and reading to average 75%:

Math     Reading
100%      50%
99%       51%
98%       52%
97%       53%
96%       54%
95%       55%
94%       56%
93%       57%
92%       58%
91%       59%
90%       60%
89%       61%
88%       62%
87%       63%
86%       64%
85%       65%
84%       66%
83%       67%
82%       68%
81%       69%
80%       70%
79%       71%
78%       72%
77%       73%
76%       74%
75%       75%
74%       76%
73%       77%
72%       78%
71%       79%
70%       80%
69%       81%
68%       82%
67%       83%
66%       84%
65%       85%
64%       86%
63%       87%
62%       88%
61%       89%
60%       90%
59%       91%
58%       92%
57%       93%
56%       94%
55%       95%
54%       96%
53%       97%
52%       98%
51%       99%
50%       100%

Edwin