What does a 2.25 GPA translate into on a 100 scale?

There's no direct conversion between a GPA and a percentage, but 2.25 is about a 72 in the US, and 2.9 is about a 78. Again, not direct conversion. As for your second question, let's assume you took 5 classes every semester - 2 D's, one C, one B, and one A is a (1.0 * 2 + 2.0 + 3.0 + 4.0)/5 = 2.2 GPA, so your overall GPA would be (2.9 * 6 + 2.2 * 1) / 7 = 2.8.