Help with redirecting a quiz based on field value totals

I have a quiz of 20 questions, 2 choices for each question, four questions to a page. I've assigned a value to each radio button choice but need some advice as how to:
(a) total up the field numbers in the functions.php; and then
(b) change the redirect URL based on a particular total.

For example, based on a user's selections, if the:
total <= 125 they go to confirmation page a
total == 130 they go to confirmation page b
total == 135 they go to confirmation page c
total >= 140 they go to confirmation page d

Thanks

Hi Chris,

Thanks for your response but need some additional help getting started.

Have looked through both topics. In the 'mathematics to gravity' understand how the rating field is used to display the message as it relates to the score. I have the same number of questions and the same number of options but the values I've put in to the field numbers are different that of '1' or '0'.

I have five pages, 4 questions to a page, 2 options per question. For example, first page:
Q1, option A, value '1'; Q1, option B, value '2';
Q2, option A, value '3'; Q1, option B, value '4';
Q3, option A, value '5'; Q1, option B, value '6';
Q4, option A, value '7'; Q1, option B, value '8';

The option values then repeat again on the following four pages.
Page 2: Q5, option A, value '1'; Q1, option B, value '2'; etc., etc., through to '8'.
Then it starts again on the next page.

Depending on what they've selected, their score should either be:
total <= 125 (message/redirect to page a)
total == 130 (message/redirect to page b)
total == 135 (message/redirect to page c)
total >= 140 (message/redirect to page d)

So, how do I total up the different values for each of the options?