Two trains having lengths of 210 m and 140 m

$ L_1 = 210,\text{m}, ; L_2 = 140,\text{m} $
Total distance = 210 + 140 = 350, m
$80,\text{km/h} = \frac{80 \times 1000}{3600} = \frac{200}{9},\text{m/s} $
$ 150,\text{km/h} = \frac{150 \times 1000}{3600} = \frac{125}{3},\text{m/s} $
$ \text{Relative speed} = \frac{125}{3} - \frac{200}{9} = \frac{175}{9},\text{m/s} $
$ \text{Time} = \frac{350}{175/9} = \frac{350 \times 9}{175} = 18,\text{s} $
$18,\text{s} = \frac{18}{60} = 0.3,\text{minutes} $
Answer = 0.3, minutes