On the 1st square: 1 grain of rice ( or 2^0 ).
On the 2nd square: 2 grains of rice ( or 2^1 )... and so on
a ) The relation is:
R ( n ) = 2^( n - 1 )
This is the number of grains on the n-th square.
This is an example of the exponential growth.
b ) For n = 64:
R ( 64 ) = 2^( 64 - 1 ) = 2^63
R ( 64 ) = 9,223,372,036,854,775,808
And if the mass of 1 grain of rice is 25 mg = 0.000025 kg
m = R ( 64 ) * 0.000025
m = 230,584,300,921,369.4 kg ≈ 2.3 · 10^(14) kg