Problem Solving (Show your calculations/steps) 

Let the unit digit is b and tenth dight is a So the number = 10a + b

According to first condition, a + b = 16 ........(i)

According to second condition, 

10a + b + 18 = 10b + a

= 10b + a - 10a - b = 18

= 9b - 9a = 18

= b - a = 2 ........ (ii)

By adding equation (i) and (ii),

a + b + b - a = 16 + 2

= 2b = 18

= b = 9

From equation (i),

a + b = 16

= a + 9 = 16

= a = 7

The original number 

= 10a + b

= $10 \times 7$ + 9

= 79 (Answer)

Dolar's 1 hour's work = $\frac{1}{4}$

(Zoha & Khadem)'s 1 hour's work = $\frac{1}{3}$

(Dolar & Khadem)'s 1 hour's work  = $\frac{1}{2}$

(Dolar, Zoha & Khadem)'s 1 hour's work $=\frac{1}{4}+\frac{1}{3}=\frac{7}{12}$

Zoha's 1 hour's work  = $\frac{7}{12}-\frac{1}{2}$= $\frac{1}{12}$

Time taken by Zoha to complete the work 12 hours  

Let cost price of the product is x ATQ,

$\frac{1920-x}{x}\times 100= \frac{x-1280}{x}\times 100$

=  1920 - x = x - 1280

= 2x = 3200

= x = 1600

At 25% the selling price

$= 1600 + 1600 \times \frac{25}{100}$

= 2000 Tk