如图所示的电路中，电源电压保持不变，电阻R₁=30Ω。当开关S₁闭合、S₂断开时，电流表的示数为0.4A；当开关S₁、S₂都闭合时，电流表的示数为0.6A。 求：

（1）电源电压；

（2）R₂的阻值。

This maths problem is _____ that one. [ ]

A. so easy as

B. as difficult as

C. much difficult than

D. less easier than