Answer: 50 tests (0r 45.5 tests if you can pass half a test)
Explanation:
We will set up an equation to help us solve. Let x be the number of tests you fail. In terms of a grade, they are usually written as a percentage, so we will assume that by 70 you mean 70%.
Given equation:

Divide both sides of the equation by 100:

Multiply both sides of the equation by 65:

Since you usually cannot pass half a test, we will round up to 50 tests.