Finding equivalent resistance

Figure:
http://session.masteringphysics.com/prob…

Find the equivalent resistance of the combination shown in the figure, R1 = 35.

Here's what I have:
R12 = 35*5 / (35+5) = 4.375 ohm
R34 = 12*25 / (12+25) = 8.11 ohm

R12+R34 = 12.485 ohm but this is wrong. This is the only formula I know atm though, please help

For parallel connection, Req = (1/R1 + 1/R2 + 1/R3 + ...) ^-1

Thus, in this question, we just have to apply the formula mention above.
Req = (1/25 + 1/12 +1/5 + 1/35)^-1 = 2.84 ohms

35||25||12||5 = [1/25+1/12+1/5+1/35]^-1 = 2.84 Ohms