Three times as many robins as cardinals visited a bird feede

[Connie]

I don't know if this is a trick question or not.

Three times as many robins as cardinals visited a bird feeder. If a total of 20 robins and cardinals visited the feeder, how many were robins?

This what I did, but I don't know how to solve my equations:

Let c stand for number of cardinals
Let r stand for number of robins

> Three times as many robins as cardinals visited a bird feeder.

So r = 3c.

> a total of 20 robins and cardinals visited the feeder

So r + c = 20.

Can you solve those two equations?

 

[pka]

3c+c=20?

 

[Connie]

This is what some 1 else explained it to me, and I got it--------> its 15

Let r represent the number of robins.
Let c represent the number of cardinals.

(1) r + c = 20
(2) c = 3r

You now have two equations in two unknowns which can be solved simultaneously.

One equation in one unknown would go like this.

Let x represent the number of cardinals.

Then 3x represents the number of robins.

Using the second statement to build your equation you could get...

x + 3x = 20.

Solve for x, then indicate what the value of 3x is.

Finally, you could just "think" it through. If there are 3 times as many robins as cardinals, you could make a table like this...

Cards Robs
1 3
2 6
3 9
4 12
5 15
6 18
etc. then find the pair of numbers that add to 20.

Thanx for your help.

 

[stapel]

Alternatively, you could have (more simply) solving the equation the tutor gave you earlier:

. . . . .3c + c = 20
. . . . .4c = 20
. . . . .c = 5

. . . . .3c = 15

Eliz.