ThinkTank #7 - Maths
Sarah sold 320 cupcakes in total.
One way we can approach solving this kind of questions is to lay out the information that the question has provided. In this case, 2/5 of the cupcakes were chocolate while 3/5 were vanilla. We can represent this using either the Model method or the Branch method.
When Sarah sold 5/6 of the vanilla cupcakes, she had 1/10 of the total number of vanilla cupcakes left. Since the question stated that Sarah had 1/5 of the total cupcakes left, this meant that Sarah also had 1/10 of the total number of chocolate cupcakes left. Thus, the 120 chocolate cupcakes that she sold represented 3/10 of the total number of cupcakes that she had.
From here, we know that Sarah sold a total of 8/10 of her total cupcakes. This means that solving (120 ÷ 3 x 8) would give us a total of 320 cupcakes sold.