Sandy's cost equation is y = 5x + 10 where y = cost and x = number of children.
Kimmi's cost equation is y = 25 where y = cost.
The fees are the same when x = 3 in other words when Sandy babysits 3 children.
That is found by solving 5x + 10 = 25 for x.
Subtract 10 from each side: 5x = 15
Divide by 5 on both sides: x = 3