Can someone please help me for 4a. i know that answer and the answer is 15d+20 but what is the answer for the second part which is 4b.??

Can someone please help me for 4a i know that answer and the answer is 15d20 but what is the answer for the second part which is 4b class=
Can someone please help me for 4a i know that answer and the answer is 15d20 but what is the answer for the second part which is 4b class=