The 7 students took a group photo, and the price was 15 yuan to wash one photo according to the regulations, and another fee was calculated for additional washing. Since each of them wanted to sign one photo, they paid a total of 36 yuan. How much would it cost to add one photo?
7位同学照合影，按规定定价15元洗l张照片，加洗另算钱．由于他们每人都想署l张照片，这样共付了36元，加洗l张照片需要多少元？
<gadget id="calculator">36 - 15</gadget>
<output>21</output>

<gadget id="calculator">7 - 1</gadget>
<output>6</output>

<gadget id="calculator">21 / 6</gadget>
<output>7/2 = around 3.5</output>

<result>7/2 = around 3.5</result>