Answer:
His friend took one from 61 and added it to 39. That made it easy to mentally add 40 and 60 to get 100. Then he mentally added 28 to 100 to get 128.
Answer:

and x=4
Step-by-step explanation:
We are given that a curve

We have to find the equation of tangent at point (4,2) on the given curve.
Let y=f(x)
Differentiate w.r.t x

By using the formula 
Substitute x=4
Slope of tangent

In given question


By comparing we get a=4
Point-slope form

Using the formula
The equation of tangent at point (4,2)




Answer:
x=6
Step-by-step explanation:
3x-7=5x-19
12=2x
x=6
Answer: imagine not knowing the answer
Step-by-step explanation: