Someone please help me. Thank you!

1) Multithreading is supposed to reduce the time of execution. Practically, when multithreading is applied on a small data set, utilizing more threads increase the execution time instead of reducing it, why?

2) by using the STL, complete the following tasks:

1. By using a stack, write a method to convert a decimal number to binary presentation (in a form of string).
2. By using a stack, write a method to calculate the final value of postfix arithmetic expression given in a form of string.
3. By using a stack, write a function to insert an element in the middle of a stack.