If it can be proved that you cheated in any of your tests, you will be ejected from the selection process. This is because almost certainly you will have agreed to the conditions attached to taking a numerical test for selection, which will have contained a declaration that you have attempted the test on your own without help, without distorting the results without deceitful intentions etc. And in the unlikely event that you didn’t have to agree to such terms, let’s face it; very few companies would want to employ a conniving candidate.
So, how do they detect cheating? Various means. Test publishers spend multiple thousands of pounds and many man-hours ensuring their tests are cheat-proof. The most common practice is to give (or pose the prospect of) a re-test under supervised conditions. So if you get a friend to help, or you employ some other deceitful means to distort your score on your online (unsupervised) test, you will probably be found out when you take a re-test in the employer’s office where you are unable to employ such tactics. At this point, if there are significant variations between your supervised and unsupervised test scores, serious suspicions of misconduct will arise.
So don’t cheat!