let's not use any eqns & try to solve using mental math
1. Cookies are divided equally among 5 members so cookies are a multiple of 5
2. Son ate his cookies share & cookies were divided again, so even after deducting son share cookies are multiple of 5
3. Now, all other 4 members have given equal share to son, basically share given is multiple of 4(4x) & share given is equal to the amount that all 5 members have now
4. Cookies eaten by each other member is multiple of 4 & becomes a multiple of 5 if we consider son share also
5. There are 2 options which are multiple of 4, option 4 & 12. for 5 persons( 4*4 +5, 12*4+12), both options are multiple of 5 if we consider 5 person
6. Although checking the options being multiple of 4 & 5 was an un necessary step, still I did
7. Now, difference between cookies eaten by son to cookies eaten by each other member will be a multiple of 5, coz when you are deducting the total cookies eaten by son to cookies eaten by each member, you are arriving to a value when son has eaten the cookies at the first time & it will be a multiple of 5 because even after reducing son share which he had eaten, the cookies were divided equally among 5 members which means that the cookies eaten by son at the first time was a multiple of 5 , only the multiple of 5 can be reduced from higher multiple of 5 to get a number which is multiple of
8. So there is no option which which gives difference a multiple of 5 if we reduced 4 from it, but for 12 there is an option 27, 27-12 = multiple of 5
IMO 12 cookies eaten by each member & 27 by Son