Let the angles be 7x, 6x, 3x, 2x. Take the sum of total angles to be 360.

Use hit and trial method to complete the equation.

Take the total tree be t and tree which produced the mangoes as (t-8), then solve the equation.

Take Nehas age as x and Ushas age as x+20. Then form the equation.

Let the age of Lokesh to be x, then form the equation for three years ago.

Take the speed of the boat in the still water as x, speed in downstream (x+3) and speed in upstream is (x-3).

Take the total tree be t and tree which produced the mangoes as (t-8), then solve the equation.

Let the additional kilometres be x and find out the value of x and then add 1 to it for the first km.

Take the tens place digit as x and ones place digit as x+4, form the equation and solve for the value of x. Notice that the value of the tens digit would be 10x.