Bob's Gift Shop sold 400 cards for Mother's Day. One salesman, Jai, sold 10% of the cards sold for Mother's Day. How many cards did Jai sell?

Divide/scale down to solve for the missing percent

Jai sold 10% of the total number of cards sold for Mother's Day. To find how many cards Jai sold, multiply the number of cards sold for Mother's Day by 10/100: 400 cards * 10/100 = <<400*10/100=40>>40 cards. Answer: \boxed{40}.