Force on left crate, right crate and truck we need to draw here
First we will write the equation for left crate
[tex]T_{left} = m*a[/tex]
[tex]T_{left} = 20*a[/tex]
Now similarly for right crate
[tex]T_{right} - T_{left} = 20*a[/tex]
now for truck
[tex]F - T_{right} = 20*a[/tex]
Now we know that F = 200 N
[tex]200 - T_{right} = 20*a[/tex]
now add all three equation
[tex]T_{left} + T_{right} - T_{left} + 200 - T{right} = 20a + 20 a + 20a[/tex]
[tex]200 = 60 a[/tex]
[tex]a = 10/3 m/s^2[/tex]
now we will find all tension using this value of acceleration
[tex]T_{left} = 20 * \frac{10}{3} = \frac{200}{3} N[/tex]
[tex]T_{right} - \frac{200}{3} = 20*\frac{10}{3}[/tex]
[tex]T_{right} = \frac{400}{3} N[/tex]