Mathematical problem solving (you must show your steps of calculation)

Mr. Salam has an open terrace adjacent to his apartment where his children play football. To protect the children from falling, Mr. Salam wants to put up railing around three sides of his rectangular terrace and leave a side of 20 feet untended as that side is covered by his apartment and needs no railing. If the terrace has an area of 680 square feet, how many feet. how many feet of railing does he need?

Problem Solving ( show your calculations/steps)

Mr. Zahid signed a contract for building a road of 1920 meters long wing 120 days. He employed 160 workers for this task . But after 24 days he found that only 1/8 that only 1/8 of the task has been finished. If Mr. Zahid wants to finish the road in time many additional workers he has to employ?

Problem Solving ( show your calculations/steps)

The simple interest on a sum of money will be Tk. 600 after 10 years. If the principal is trebled after 5 years , what will be the total interest at the end of the tenth years?

Mathematical Problems (Show your steps/calculation)

A scientist has 400 units of a 6% phosphoric acid solution, and an unlimited supply of 12% phosphoric acid solution. How many units of the latter must she add to the former to produce a 10% phosphoric acid solution?

Mathematical Problems (Show your steps/calculation)

A school store sells only one kind of pen and pencil at a uniform price per pen or per pencil. If the cost of 3 pens and 6 pencils is twice that of 12 pens and 4 pencils, then the total cost of 2 pens and 4 pencils is how many times that of one pen?