There are

#1.00color(white)(l)color(red)(cancel(color(black)("hours")))*(60.0color(white)(l)color(purple)(cancel(color(black)("minutes"))))/(1.00color(white)(l)color(red)(cancel(color(black)("hours")))) cdot (1color(white)(l) "halflife")/(20.0color(white)(l)color(purple)(cancel(color(black)("minutes"))))=color(navy)(3)color(white)(l) "halflives"#

in #1.00color(white)(l)"hours"# of time.

The mass of the sample halves every halflife of the decay. That is: given an initial mass of #m_0#, the remaining sample will have mass

#m=(1/2)^(n)*m_0#

after #n# halflives.

Therefore, the sample will have a mass of

#m=(1/2)^color(navy)(3)*1.00color(white)(l) "g"#

#color(white)(m)=1/8*1.00 color(white)(l) "g"#

#color(white)(m)=0.125 color(white)(l) "g"#

after an hour.