College Physics (7th Edition)

$2m/s^{2}$
Mass of first block =$\frac{F}{a}=\frac{40N}{2.5m/s^{2}}=16kg$ Mass of second block =$4kg$ Combined mass$=16+4=20kg$ So, acceleration of combined mass $=\frac{40N}{20kg}=2m/s^{2}$