NL-NWO 1995 Problem 3

Let $101$ marbles be numbered from $1$ to $101$ . The marbles are divided over two baskets $A$ and $B$ . The marble numbered $40$ is in basket $A$ . When this marble is removed from basket $A$ and put in $B$ , the averages of the numbers $A$ and $B$ both increase by $\frac{1}{4}$ . How many marbles were there originally in basket $A?$