x=nr elevilor prezenti in prima zi
x/13=nr elevilor absenti in prima zi
x/13 - 1 =nr elevilor absenti a 2-a zi
x+1 =nr elevilor prezenti a 2-a zi
x/13-1=1/20(x+1)
x/13 -1=x/20-1/20
20x-260=13x-13
7x=273
x=39 elevi
39/13=3 elevi absenti in prima zi
39+3=42 elevi in total