If you had a pizza with crust thickness 'a' and radius 'z', what's the volume of the pizza?

In a competitive exam, each correct answer could win you 10 points and each wrong answer could lose you 5 points. You sat in the exam and answered all the 20 questions, which were given in the exam.
When you checked the result, you scored 125 marks on the test.

Can you calculate how many answers given by you were wrong?

What happened when the wheel was invented?

Look at this sequence from top to bottom. What is the next number in the sequence?
1
11
21
1211
111221
312211

81 x 9 = 801. What must you do to make the this equation true?

There are three boxes of different sizes with three cupcakes inside each of the boxes in Christina's kitchen. At night her daughter wakes up and goes to the kitchen. She opens a box and eats all three cakes.

But in the morning Christina finds out that each box still had three cupcakes. How?

What jumps when it walks and sits when it stands?

How do you go from 98 to 720 using just one letter?

I am 5 letters long.
My first two tell you who I am
My first 3 could be a medicine
My last three reversed could be a young boy.
My 4th, 3rd and 2nd in that order could be a fruit drink.
If you have me you may hang me round your neck
WHAT AM I ?

I have branches, but no fruit, trunk or leaves. What am I?