The first 1 in 1 contains two 1's , both formed by adding the two numbers above them to the left and the right in this case 1 and 0(all numbers outside the triangle are 0's).Do the same to create the second row :0+1 =1;1+0=1;1+1=2;1+0=1;And the third row you put:0+1 =1;1+2=3;2+1 =3;1+0=1