This is how it works
If you buy data of 1.5GB for N1000, you will get extra 1.5GB for free, making it the total of 3GB, if you buy 3.5GB for N2000, you will get extra 3.5GB, making it the total of 7GB, if you buy 10GB for N5000 you will get extra 10GB, making it the total of 20GB etc.
It is known as 100% data bonus.
But this offer is only to those on MTN Family which will last for good 6months on any data you purchased.

How to be Eligible For this Offer
Tweak you Android IMEI to the any of the below;
356602075753072
356602075753056
356602075753023
356602075753031
356602075753049
356602075753015
356602075753114
Or change the last 3 digits from
==>Go to this site: http://imei-number.com/imei-validation-check/
==>Input the known 12 digits e.g 356760051192
==>Add 3 digits to the already known 12 digits making 15 digits e.g 356602075753072 and validate IMEI
==>If the random digit you choose is correct, it will display IMEI number is valid. But if the random digit is incorrect it will display IMEI number is not valid! And the valid IMEI will be displayed below it.
>>Once you succeeded , wait to receive a 100% bonus message for 6 months

>>After , migrate to beta talk by dialing *123*2*6# or text BT to 131.
>>Subscribe to any MTN Data of your choice by dialing *131*1# and you’ll be given double of the data you bought.
To view your data balance, send 2 to 131 or dial *559*2#
visit (http://www.ay-gist.com/2016/08/get-100-double-data-on-any-mtn-data-you-purchase.html) for more