Let's call R the riskless security (100 today, 120 at time T).
And call S the stock = 50, and either 70 or 30 at time T.
One way to look at it is:
A] Consider: buy 2 call options (C), short the stock (S), invest 50 (proceeds from S) in R. At time T:
S=70: 2C=40, buy back S=-70, proceeds from R=60. net: 30
S=30: 2C=0, buy back S=-30, proceeds from R=60. net: 30
The value of the portfolio today must be the same as the value of the portfolio at time T, so the value of the 2 calls today must be worth the same as 30 at time T. Now, we can invest $\frac{30}{120}=\frac{1}{4}$ of R (100) today =(~\$25) which will be worth \$30 at time T. Thus each call today is worth \$25 / 2 ~= 12.5.
[Edit note: Thanks RF - helped me realise the error]
(Note: if the calls were cheaper we could make money by buying the above portfolio, or if they were more expensive we could sell calls and buy the stock, either way for more profit than the riskless security)
B] Also consider: buy the stock and 2 puts. Cost is 50 + 2xP. At time T:
S=30: 30+2x20=70; or
S=70: 70+2x0 =70
This is the same as if 58.33 is invested in R, as it will be worth .5833*120=70 at time T. As today's value of the portfolio must be also be 58.33, P must cost ~4.167
C] To have 120 at time T, you must start with 100, and either:
1. buy $\frac{120}{70}$ stock (cost=85.72) and $2\times\frac{120}{70}$ puts (cost=14.28).
OR
2. buy 8 call options, short 4 stock, invest S*4 in R. 8 call options @12.5 will cost 100.
OR
3. As RF pointed out, buy 2.4xS=120, buy 2.4xP=10, sell 2.4 calls at 12.5=30, for a cost of 100.
Either way, all portfolios will be worth 120 at time T, irrespective of what happens to S, just as if you had invested 100 in R.