A stack of one hundred eighty cards is placed next to a ruler, and the height of stack is measured to be 3 8 38 inches. how thick is one card?

Total number of cards in the stack = 180

Height of stack (all cards) = 38 inches

Now, we know that the thickness of 180 cards is 38 inches,

now the thickness of one card can be obtained by dividing the total thickness by the number of cards

Thickness of 1 card = Total thickness of all cards / total number of cards

Thickness of 1 card = 38 inches/180 = 0.211111 ... inches

Each card is 0.21 inch thick (rounded to two decimals)