75. 2 times $a$ plus 2 equals $b$; 2 times $b$ plus 2 equals $c$; 2 times $c$ plus 2 equals $d$; 2 times $d$ plus 2 equals 62. Then, $a=$ ـ. $\qquad$
Reference answer: 2
Key point: Reverse inference



Translate the text above into English, please retain the original text's line breaks and format, and output the translation result directly.