The separations between the lenses will be as follows,
Distance between L2 and L3 doesn’t matter as the image formed by L2 is at infinity so that X2 can take any value.
Image by L3 is formed at the focus. So the object should lie at infinity for L3.
So, L2 will form image at infinity and the object for L2 should be at its focus.
But wehave seen above that the image by L1 is formed at 20cm right of L1 which is at 20cm left of L2.
So, Distance betweenL1+L2=15+20=35cm