t=tables
b=booths
t+b=27
8t+4b=140
solve
8t+4b
divide both sides by 4
2t+b=35
we have
t+b=27
2t+b=35
it should be obvious that the answer is 8 but using math
multipl first eqution by -1 and add to second
-t-b=-27
2t+b=35 +
1t+0b=8
t=8
sub
t+b=27
8+b=27
minus 8 both sides
b=19
19 booths