You do 15/4 MULTIPLIED by 2/5 (you change the division sign to a multiplication sign) then you multiply the fractions : 15*2=30 and 4*5=20 so it would equal to 30/20 which can be halved to give 15/10
The slope intercept form is y=mx+b, and b represents the y-intercept. In the given equation C=45t+100, 100 is the y-intercept. It represents the one-time fee to join the gym membership.