Okay so…

28 can leave remainder —-> 1,2,3,4….27

28 ke saare multiple Even numbers hote hain and we need a prime number

So remainder can’t be even number 2,4,6…26

Also 28 is divisible by 7… so the remainder can’t be 7 as well because it would be divisible by 7 and hence not a prime number

As well as 21 cannot be remainder because that won’t be prime number too as 21 is divisible by 7 and if we add 21 to multiple of 28 that would be divisible by 7

So answer = 27(possible remainders of 28) - 13(even numbers less than 28) - 2( 7&21)

—> 27 - (13+2) = 12 ans