i want that the computer finds my secret number which i keep in mind [on hold]

hello guys i have a problem. i want that the program guess with the 'srand(time(NULL))' command which number i have i mind. for example if i have the number 30 in mind and the computer gives me 57, then i'll command the program to give a lower number , when the program gives 20 i'll command the program for an upper number. but the problem is how can i narrow the intervals of the guessing process ? and by the way im a new c++ learner i dont understand much of cin and cout commands could you maybe help me with printf scanf commands?