The simple interest is calculated as :
S.I = P*T*R
where S.I is the simple interest to be calculated
P = Principal
T = Time period in years
R = Annual interest rate
The the above problem,
Amount borrowed which would be the principal (P) = $400
Time period = 6 months = 1/2 year
Annual interest rate = 6.8% = 6.8/100 = 0.068
Simple Interest = P*T*R
Simple Interest = 400*1/2*0.068 = 200 *0.068
Simple Interest = $13.6
The simple interest amount that Miguel must pay to Julio at the end of six months is $13.6