The positive integers which are divisible by 6 are:

6,12,18,24,................up to 40 terms

It forms an arithmetic series. 

Now

first term a = 6

common difference d = 6

number of terms n = 40

Now sum of first 40 terms of AP = (n/2) *{2a + (n-1)*d}

                                               = (40/2)*{2*6 + (40-1)*6}

                                               = 20*(12 + 39*6)

                                               = 20*(12 + 234)

                                               = 20*246

                                               = 4920

So sum of first 40 terms which are divisible by 6 = 4920