a. B are different prime numbers, and 1 / A + B / a = 20 / 91, what are a and B?

a. B are different prime numbers, and 1 / A + B / a = 20 / 91, what are a and B?

It is wrong to estimate the title. The correct one should be as follows
1/a＋1/b=20/91
Then 20ab = 91 (a + b) = 13 * 7 (a + b)
So 10ab = 13 * 7 * [(a + b) / 2]
a. B are different prime numbers, so (a + b) / 2 is not equal to a, (a + b) / 2 is not equal to B
So (a + b) / 2 = 10
So AB = 13 * 7
So, a = 13, B = 7 or a = 7, B = 13

If a is prime and a + 18 is prime, what is the minimum of a?

A the minimum is 5
5 is prime
5 + 18 = 23 is also prime

If the sum of three times of a prime number and two times of another prime number is 100, what is the sum of the two prime numbers?

Two times of a prime number must be even, 100 minus two times of the prime number is even, that is to say, three times of a prime number is even, that is to say, the prime number is even, that is, 2, (100-2 * 3) / 2 = 47, so the other prime number is 47, and 47 + 2 = 49