You can solve this by setting up a proportion. You can put the number of items correct over the total number. and substitute the unknown number with x.
= 
Cross-multiply
3400 = 100x Divide both sides by 100
34 = x
40 - 34 = 6
So, Jill got 6 questions wrong.
3400 = 100x Divide both sides by 100
34 = x
40 - 34 = 6
So, Jill got 6 questions wrong.