Quantitative Aptitude - Arithmetic Ability Questions

Let the ten's digit be x and unit's digit be y.

Then, (10x + y) - (10y + x) = 63  

=>  9 (x - y) = 63

=> x - y = 7.

Thus, none of the numbers can be deermined..

As per given 

 40100×250 =50100×x

⇒x=200

Prime Numbers :: Numbers which are divisible by only 1 and itself are Prime Numbers.

It's answer will be 91.

Because 91 can be divisible by 7,13,91,1.

It is quite clear that prime number should be divisible only by itself and by 1.

Total number of exams written by 100 students = 48 + 45 + 38 = 131
Now let us say x members are writing only 1 exam, y members are writing only 2 exams, z members are writing only 3 exams.
Therefore, x + 2y + 3z = 131 also x + y + z = 100.
Given that z = 5. So x + 2y = 116 and x + y = 95.
Solving we get y = 21.
So 21 members are writing exactly 2 exams.

Since the numbers are co-prime, they contain only 1 as the common factor.
Also, the given two products have the middle number in common.
So, middle number = H.C.F of 551 and 1073 = 29;
First number = 551/29 = 19
Third number = 1073/29 = 37.
Required sum = 19 + 29 + 37 = 85.