a) 24F = (130 - 10)
b) By inspection...I see that the right side evaluates to 120....and I know that 24 divides 120 five times.....so...there are 5 friends
By.....systematic trial.....we could plug 1,2,3,4 and 5 in for F and find that 5 would work
The second method is easier, in my opinion, if you don't happen to see that 24 divides 120