if(text[0] == nra+nrb+nrc) {
      new string[128]; new stringy[128]; GetPlayerName(playerid,stringy,sizeof(stringy));
        format(string,sizeof(string),"%s has won the Reaction Test. Solution: %d",stringy,text[0]); SendClientMessageToAll(yellow, string);
      return 0;
    }

//on top of your script: 
forward Result();
new result12[128];
new calculated;

//ongamemodeinit:
SetTimer("Result",180000, 1); //timer is set to 3mins, change it 

//end of your script:
public Result()
{
    new random1,random2,random3,str[128];
    random1 = random(999);
    random2 = random(999);
    random3 = random(999);
    format(str,sizeof(str),"[Calculation] calculate: %d + %d - %d and win 5000$",random1,random2,random3);
    SendClientMessageToAll(0xFFFFFF,str);
    printf("%d+%d-%d",random1,random2,random3);
    format(result12,sizeof(result12),"%d",random1+random2-random3);
    calculated = 1;
    return 1;
}

//onplayertext:

for(new i=0; i<256; i++)
{
        if(strfind(text[i],result12) != -1 && calculated == 1)
        {
            
            SendClientMessageToAll(THIAZ_YELLOW,"the calculation has been finished!"); //do this for yourself!
            GivePlayerMoney(playerid,5000);
            calculated = 0; //set this to 0, so that noone else can write the result after now.
        }
    }