$25.$

$\mathscr{P_1}=\dfrac{U_1^2}{R}=\dfrac{6^2}{R}=\dfrac{36}{R}$

$\mathscr{P_2}=\dfrac{U_2^2}{R}=\dfrac{12^2}{R}=\dfrac{144}{R}$

$\dfrac{\mathscr{P_1}}{\mathscr{P_2}}=\dfrac{\dfrac{36}{R}}{\dfrac{144}{R}}=\dfrac{1}{4}$

$⇒\mathscr{P_2}=4\mathscr{P_1}=4.1=4W$

 $26.$

$U_1=5V$

$U_2=U_1+20\%U_1=6V$

$\mathscr{P_1}=\dfrac{U_1^2}{R}=\dfrac{5^2}{R}=\dfrac{25}{R}$

$\mathscr{P_2}=\dfrac{U_2^2}{R}=\dfrac{6^2}{R}=\dfrac{36}{R}$

$\dfrac{\mathscr{P_1}}{\mathscr{P_2}}=\dfrac{\dfrac{25}{R}}{\dfrac{36}{R}}=\dfrac{25}{36}$

$⇒\mathscr{P_2}=\dfrac{36}{25}.\mathscr{P_1}=\dfrac{36}{25}.5=7,2W$

$∆\mathscr{P}=\mathscr{P_2}-\mathscr{P_1}=7,2-5=2,2W$

$27.$

$A=\mathscr{P}.t=3,75.3600=13500J$