The school bought some skipping ropes and distributed them to the sixth and fifth grades at a ratio of 5:4. It is known that the sixth grades have 4 more than the fifth grades. How many skipping ropes did the school buy?
学校买来一些跳绳，按5：4发给六年级和五年级，已知六年级比五年级多得4根，学校买来多少根跳绳？
<gadget id="calculator">5 - 4</gadget>
<output>1</output>

<gadget id="calculator">4 / 1</gadget>
<output>4</output>

<gadget id="calculator">5 + 4</gadget>
<output>9</output>

<gadget id="calculator">4 * 9</gadget>
<output>36</output>

<result>36</result>