There are 11 sheets of RMB, including 1 sheet for 10 yuan, 2 sheets for 5 yuan, 3 sheets for 2 yuan, and 5 sheets for 1 yuan. Now buy books for 20 yuan. What is the number of payment methods.
有人民币11张，其中10元1张，5元2张，2元3张，1元5张，现买20元的书．付款方法的种数是多少．
<result>6</result>