120
Explanation:
turning $54 to cents
$1= 100c
$54=54×100= 5400
calling the number of 20c coins a and 50c coins b
20a + 50b= 5400 ...equ(1)
since there are twice as many 20c coins as 50c coins
a=2b ...equ(2)
substituting a=2b in equ(1)
20(2b) + 50b = 5400
40b + 50b = 5400
90b = 5400
dividing both sides by 90
b= 60
to get the number of 20c coins I'm substituting b=60 in equ(2)
a= 2×60
a=120
therefore the number of 20c coins is 120