Answer:
21 mails
Step-by-step explanation:
Let there are x no of texts and y no of emails.
Condition 1,
x+y = 20 ...(1)
Condition,
x=2y-1 ...(2)
Put the value of x from equation (2) in (1). So,
(2y-1)+y = 20
Taking like terms together,
2y-y=20+1
y=21
So, Devin sent 21 mails,.
X - speed of the current
40+x - speed with current
40-x - speed against current
t-time
(40+x)*t=100
(40-x)*t=60
40t+xt=100
<span> + 40t-xt = 60
</span> 80t =160
t = 2
40t+xt=100,
40*2+2x=100
80+2x=100
2x=100-80
2x=20
x=10 mi/h
10 mi/h speed of the current.
