We know that the masses experience a 40 N force between them, which is a gravitational force due to their mass. So, if we double their distance between them, the force will decrease due to Newton's Gravitational Law.
[tex]F=G\cdot\frac{m_1\cdot m_2}{d^2_{12}}[/tex]Let's use 2d.
[tex]F=G\cdot\frac{m_1\cdot m_2}{(2d)^2}=G\cdot\frac{m_1\cdot m_2}{4d^2}[/tex]As you can notice, the force would be divide by 4, so let's do that.
[tex]F=\frac{40N}{4}=10N[/tex]