Thinking Mathematically Problem Solving

It was a fun to solve this problem bcs it was like a game of numbers to win. The problem is to check a number best to start when playing in pair of two . The numbers given are 1,2,34,5
the players alternately name a number .The winner will be that person who first brings a combined total of 31.
2.Can you find a strategy.
3. what if 31 changes to another number.
4. What if numbers are 1,3,5 or 2,3,7.


I figured out total of digits 1,2,3,4,5 is 15 so I thought 15+15=30.i.e two full rounds of all digits and 30+1 is the game to win.In other words if I start announcing 1 as a first number then a total of 31 can be achieved at the end with 1.
Again if total changes to 33 then I can start with 2 and so on for 35 or 37 .........also for 29, 27.............. At the same time if digits change to 1,3,5 I was able to follow the same strategy of starting with 1 for a total of 31.Bcs every time it follows a sequence of choosing number from a fixed group of numbers. So it looks to me as a fixation of numbers.I found it is the best strategy only if the players have to select all digits differently in all rounds.If all digits can be repeated the I can go simply choosing 24+7, 26+5, 25+6 or 28+3 and so on..................... but then second player can also choose the last number to announce by figuring out the total before the last number.