Level 0 = 1 node, Level 1 = 2 nodes, Level 2=4 nodes, Level 3=8 nodes, Level 4=16 nodes, Level 5=32 nodes.
So, level 4 alone cannot give us 20 leaves, but level 5 if taken wholely will give 32 leaves but we need only 20.
So, we need some nodes of level 4 to have children and other nodes to not have children and stay as leaves.
The equation for it will be:
(16-x)*2 +x = 20
or, x = 4
so all nodes till level 3 have two children each to their full capacity and therefore 1+2+4+8 = 15 nodes have two children each till level 3.
Now, 4 nodes in level 4 will have children too, making the count of nodes with two children = 15+4 = 19.