Digital call price from a digital put at 0.36

A digital put pays $1$ if $S_T<K$ and nothing otherwise; the matching digital call pays $1$ if $S_T\ge K$. The digital put costs $0.36$ and a bond paying $1$ at expiry costs $0.9$. What should the digital call cost?

$0.54$. Together the digital call and put pay $1$ in every state, exactly like the bond, so their prices sum to $0.9$ and the call costs $0.9-0.36=0.54$.