Can someone answer these with steps??

If 1/2+2/5s=s-3/4,what is the value of s? a. s=3/4 b. s=25/12 c. s=-25/12 d. s=-3/4

International calls require the use of a country code. many country codes a 3-digit numbers. country codes do not begin with a 0 or 1. there are no restrictions on the second and third digits. how many different 3-digit country codes are possible?

Find the pattern and use it to list the nth term in the sequence. 6, 12, 20, 30, 42, 56, the nth term of the sequence is