A freshly isolated sample of ${?}^{90}Y$ was found to have an activity of $6\mathrm{.}9{10}^5$ disintegrations per minute at $1$:$00$ p$.$m$.$ on December $3,2006.$ At $2$:$15$ p$.$m$.$ on December $17,2006,$ its activity was measured again and found to be $1\mathrm{.}8{10}^4$ disintegrations per minute. Calculate the half$-$llfe of ${?}^{90}Y.$ Round your answer to $2$ significant digits.