Two masses M and m are connected by a weightless string. They are pulled by a force F on a frictionless horizontal surface. The tension in the string will be?

$\frac{F(M+2m)}{m+M}$

Whole system is moving with an acceleration a = $\frac{F}{m+M}$

For mass m equation of motion is T - F = ma = m $\frac{F}{m+M}$

⇒ T = F + m $\frac{F}{m+M}$ = $\frac{F(M+2m)}{m+M}$