(i) Present age of Sanchi = 12 years
Present age of Sam = 2 years
Suppose, Sanchi becomes three times as old as Sam in x years.
So, 12 + x = 3(2 + x)

12 + x = 6 + 3x

6 = 2x

x = 3
(ii) Both trains are travelling towards each other.
So, relative speed = 90 + 60 = 150 km/hour
Distance = 750 km
∴ Time taken by the trains to pass each other =

= 5 hours
(iii) Speed of truck = 40 km/hr
So, distance covered by truck in 5 hours = 40 × 5 = 200 km
The car covered 200 km in 4 hours.
∴ Average speed of car =

= 50 km/hr
(iv) Sum of the digits is 7. Let the unit digit of the number be x.
So, tens digit of the number = 7 - x
∴ Number formed is 10(7 - x) + x = 70 - 9x
On reversing the digits, number formed is 10x + (7 - x) = 9x + 7
According to question, we have:
9x + 7 = 70 - 9x - 9

18x = 54

x = 3
So, the required number is 43.