%% in r language gives the modulus of two numbers. It is basically the remainder of division of first number by second number.
Consider this code –
a = 5 b = 2 a %% b [1] 1
Here we are doing 5 %% 2
which says that we want the remainder of 5 / 2
. So,

5
= Dividend 
2
= Divisor 
5 / 2
= Quotient 
5 %% 2
= Remainder
And according to mathematics –
Dividend = ( Divisor x Quotient ) + Remainder
So, a = (b x int(a / b)) + (a %% b)