In the decay chain U-238 to Pb-206, how many alpha decays occur? -

The half-life of a radioactive substance is 10 days. The fraction remaining after 30 days is: <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 300 120" style="max-width:300px;margin:0.5em auto;display:block;"> <title>Decay over 3 half-lives: N0 to N0/2 to N0/4 to N0/8</title> <rect width="300" height="120" fill="#faf9f5" rx="6"/> <defs><marker id="ah-q026" markerWidth="7" markerHeight="5" refX="7" refY="2.5" orient="auto"><polygon points="0 0, 7 2.5, 0 5" fill="#14140f"/></marker></defs> <line x1="30" y1="90" x2="280" y2="90" stroke="#14140f" stroke-width="1.5" marker-end="url(#ah-q026)"/> <line x1="30" y1="90" x2="30" y2="10" stroke="#14140f" stroke-width="1.5" marker-end="url(#ah-q026)"/> <text x="265" y="105" font-size="9" font-family="sans-serif" fill="#14140f">t(days)</text> <text x="8" y="18" font-size="9" font-family="sans-serif" fill="#14140f">N</text> <path d="M30,20 Q80,30 110,55 Q150,72 200,82 Q240,87 270,89" fill="none" stroke="#c6613f" stroke-width="2"/> <circle cx="30" cy="20" r="2.5" fill="#c6613f"/> <text x="8" y="23" font-size="8" font-family="sans-serif" fill="#14140f">N&#8320;</text> <line x1="100" y1="55" x2="100" y2="93" stroke="#788c5d" stroke-width="1" stroke-dasharray="3,2"/> <text x="92" y="105" font-size="8" font-family="sans-serif" fill="#788c5d">10</text> <text x="5" y="58" font-size="7" font-family="sans-serif" fill="#14140f">N&#8320;/2</text> <line x1="160" y1="72" x2="160" y2="93" stroke="#788c5d" stroke-width="1" stroke-dasharray="3,2"/> <text x="152" y="105" font-size="8" font-family="sans-serif" fill="#788c5d">20</text> <line x1="220" y1="82" x2="220" y2="93" stroke="#788c5d" stroke-width="1" stroke-dasharray="3,2"/> <text x="212" y="105" font-size="8" font-family="sans-serif" fill="#788c5d">30</text> <text x="225" y="78" font-size="8" font-family="sans-serif" fill="#c6613f">N&#8320;/8</text> </svg> -

In alpha decay of _92^238 U, the daughter nucleus is: -

The activity of a radioactive sample is 200 Bq. After 4 half-lives, the activity is: -

The fraction of a radioactive sample that decays in two half-lives is: -