Step-by-step explanation:
the potential cards with multiples of 5 are :
5 10 15 20
the sum of all 4 cards is less than 40.
and exactly 3 of the cards are even. that means the 4th card is odd or uneven.
exactly 3 cards are a multiple of 5. that means the 4th card is something else.
what chances do we have to pick from the multiples of 5 ?
can we pick the 2 uneven ones ? no, that would violate the even card condition.
so, we must pick the 2 even ones : 10, 20
they are fixed.
now, 10+20 = 30, so, we cannot pick 15 as the third multiples of 5 card, or we violate the less than 40 rule.
so, we can only pick 5 as the third multiple of 5 card.
now, we have already 5, 10 and 20 (in sum 35).
the 4th card must be even (so that we get 3 even number cards), and it must be small, so that the total sum stays less than 40.
that means we can only pick 2 or 4 as the 4th card.
therefore, we have the 2 possibilities of picked cards :
2, 5, 10, 20
4, 5, 10, 20
