X+Y+Z=20 .............. equation 1
8X-5Z= 105 .............. Equation 2
Equation 1*5, plus Equation 2:
5X+8X+5Y+5Z-5Z=20*5+ 105
13X+5Y=205
Y=4 1- 13X/5
Therefore, x must be a multiple of 5.
And x must be greater than 10 (because the score of 105 is greater than 10*8=80) and less than 20 (because the score of 105 is less than 20*8= 160), so x can only be/kloc-.
y = 4 1- 13 * 3 = 2z = 20- 15-2 = 3。
Verification:
15*8-3*5= 105, so the answer is correct.
Do 15 right, do 3 wrong, don't do 2.