here just u hav to do

for 8-bit 2^8 256

251 is the largest prime num

for 6-bit 2^6 64

61 is the prime num

for 7-bit 2^7 128

127 is the largest primeno

for 1st machine 100-25 ie 75
for 2nd machine 100-20 ie 80
for 3rd machine its 100-10 ie 95
so efficiency in rev (90*80*75)/10000
54
so fuel economy 100-54
46