b)
n congruency sign 1 (mod 7) n=7p+1
n congruency sign 7 (mod 13) n=13q+7
n congruency sign 13 (mod 20) n=20r+13
13,33,53,73,93,113,133,153,173,193,...............293
which is a multiple of 7
12,32,52,72,92, 112, ....... ...................................292
which is a multiple of 13
6,26,46,66,86,106,126,146,166,186,206,226, 246,266,286,
33+13*20n = 33+260n will satisfy 13(mod20) and 7(mod 13)
33,293,553,813,1073
33+260n which of these will be 1 mod 7
32+260m must be a multiple of 7
32+260m must be a multple of 7
260m+32=7k
292, 552, 812,
812+1=813
813 = 1(mod 7)
813 = 13(mod 20)
813 = 7(mod 13)
The smallest number that meets those criterion is 813
There is probably a much better way to do it though ://