"Sini was 12years older than Sana 4 years ago": A- 4= B- 4+ 12 which reduces to A= B+ 12. (If Sini was twelve years older than Sana at any time he alway will be 12 years older!)
In four years time, Sini's age will be A+ 4 and Sini's age will be B+ 4.
"He will be twice as old as Sana in 4 years time." A+ 4= 2(B+ 4) which is the same as A+ 4= 2B+ 8 or A= 2B+ 4. Subtract A= B+ 12 from that to get 0= B- 8 so B= 8. Sana is 8 years old and Sini is 8+ 12= 20 years old. Three years ago Sana was 8- 5= 3 years old and Sini was 20-3= 17 years old The sum of there ages three years ago is 17+ 3= 20.
Let the length of the rectangle be "L" and the initial width be "W". Then its initial area is LW. If the length is increased by 16% it will become L+ 0.16L= 1.16L. If its width is decreased by 16% it will become W- 0.16W= 0.84W. The new area will be (1.16L)(0.84W)= 0.9744LW so that the change is 0.9744LW- Lw= -0.0256 LW or a decrease of 2.56%.2) The length of a rectangle is increased by 16% and its width is decreased by 16%. Find the percentage increase or decrease in the area.
3) During a cross-country race, Newson ran 50% of the journey at 10km/h,Suppose the entire race was of length "L" km. Then he ran .5L at 10 km/h which took .5L/10= L/20 hours. He ran .25L at 7km/h which took .25L/7= L/28 hours. He ran the remaining .25L at 5 km/h which took .25L/5= L/20 hours. That is, he ran the total L km in L/20+ L/28+ L/20= 19L/140 hours or an average speed of L/(19L/140)= 140/19 km/h.the next 35% at 7km/h and remaning journey at 5km/h. Calculate the average speed in km/h of Newson for the whole journey.