Since we know how wide the garden is (10 feet) we can also assume that one of the others sides is also 10 feet. with this information, we can subtract it from the overall perimeter (50 - 20 = 30). Since we know the unknown other two sides are going to be equal as well, we can divide 30 by 2 and get our missing side (15 feet). We can complete the problem by taking 15 x 10 = 150ft2 :)