Diego baked 12 cakes for his sister's birthday. Donald also baked 4 cakes, but ate 1 while waiting for the party to start. How many cakes are left?
The total number of cakes baked is 12 + 4 = 
<gadget id="calculator">12+4</gadget>
<output>16</output>
16 cakes.
But since Diego ate 1 cake, there are 16 - 1 = 
<gadget id="calculator">16-1</gadget>
<output>15</output>
15 cakes left.

<result>15</result>