[ikhlas]

Hi guise,

Please any one could teach me how can i find the formula or good way to solve reminder,, I have some questions and i like to solve these as math way ?
Please any help,
When n is divided by 7 , the remainder is 5 . What is the remainder when 2n is divided by 7 ?
the answer is :- I have to think for any number which leaves a reminder of 5 when dived by 7 but if I hadn't this idea ,, could we solve this problem in other way please ???
Another one in the attachment?
Thanks in advance for any help
Ikhlas