First, convert the percentages into fractions. Since Julia misses 50% of her first shots, she misses 1/2 of her first shots. Since she misses 20% of her second shots (after missing the first one), she misses 1/5 of her second shots.
To find the probability of missing two shots in a row, multiply the two fractions together:
1/2 x 1/5 = 1/10
1/10 correlates to 10%. So, Julia has a 10% probability of missing two shots in a row.
