I'm playing catch-up here as I'm on holiday in the Canaries (Isles of dogs, not to be confused with Canary Wharf in London which is on the area previously known as "the isle of dogs"). So:

If C is lying then the number IS 150 but that would mean that A is also lying but only one person is lying.
No, the question states " You now have all the information needed to figure out the number EXCEPT one of them is lying. it doesn't actually state that only one of them is lying! (a small point, but badly phrased questions permit multiple correct answers!)

Bert: Actually, In England we used to call mathematics sums.
Strange, in Wales we called Arithmetic 'Sums'. Mathematics (hence the plural 'maths') was the three mathematical disciplines of Arithmetic, Geometry & Algebra.

1 • How much?
Suppose you and I have the same amount of money. How much must I give you so that you have ten dollars more than I? 5 dollars

2 • The Politician Puzzle
A certain convention numbered one hundred politicians. Each politician was either crooked or honest. We are given the following two facts:
(1) At least one of the politicians was honest.
(2) Given any two of the politicians, at least one of the two was crooked.
Can it be determined from these two facts how many of the politicians were honest and how many were crooked? One honest & 99 dishonest. (We may be wrong about the one though!)

3 • Old Wine in a Not-so-new Bottle
A bottle of wine cost ten dollars. The wine was worth nine dollars more than the bottle. How much was the bottle worth? 50cents