In this task, you need to answer the given multiple-choice question on the physics. Classify your answers into 'a', 'b', 'c', 'd', and 'e'.

Problem: a man can do a piece of work in 4 days , but with the help of his son he can do it in 3 days . in what time can the son do it alone ? 
Options: a ) 12 days , b ) 6 days , c ) 5 days , d ) 4 days , e ) none of these
a

Problem: 28 machines can do a work in 10 days . how many machines are needed to complete the work in 40 days ? 
Options: a ) 10 , b ) 6 , c ) 4 , d ) 7 , e ) 5
d

Problem: a train 360 m long runs with a speed of 45 km / hr . what time will it take to pass a platform of 240 m long ? 
Options: a ) 38 sec , b ) 35 sec , c ) 44 sec , d ) 48 sec , e ) none of these

d