Answer to b
11.75D
If the refraction is more than 4D, the back vertex distance
needs to be taken into consideration due to lens effectivity. 
For the plus lens, the power is lost as it is moved towards
the eye. Therefore, the required contact lens needs to be 
stronger. The required contact lens power can be calculated 
as follow:
1/10D =100mm
100mm - 15mm = 85mm =0.085mm
1/0.85 =  11.76 D and round off to 11.75 D (because the contact lens come in step of 0.25)