The answer is the remainder when that number is divided by 7. For example 10 is equal to 3 mod 7 since 10 divided by 7 is 1 remainder 3. The traditional way to write that is 10 = 1*7 + 3
I've highlighted the remainders in red below
15(mod 7) = 1(mod 7) since 1=0*7 + 1
25(mod 7) = 32(mod 7) = 4(mod 7) since 32 = 4*7 + 4
35(mod 7) = 243(mod 7) = 5(mod 7) since 243 = 34*7 + 5
45(mod 7) = 1024(mod 7) = 2(mod 7) since 1024 = 146*7 + 2
55(mod 7) = 3125(mod 7) = 3(mod 7) since 3125 = 446*7 + 3
65(mod 7) = 7776(mod 7) = 6(mod 7) since 7776 = 1110*7 + 6