Question 752550
Let x = the number of questions worth two points each.
The remainder (that's 30-x) are worth three points each.
The sum of these is 78 points.
So we can write the equation:
{{{2x + 3(30-x) = 78}}} Solve for x.
{{{2x + 90-3x = 78}}} Combine the x's.
{{{-x + 90 = 78}}} Subtract 90 from both sides.
{{{-x = -12}}} so...
{{{x = 12}}} 
There are 12 two-point questions and 18 three-point questions.